Fann means art, skill, talent. Fankaar is anyone who has a talent or a skill or a lot of expertise in a certain form of art; an artist, a poet, a writer etc

Kuchh toh hai baat ke tehreeron mein taaseer nahi
Jhootein fannkaar nahi hai toh qalam jhootein hai

tehreer– lines written (as in a couplet)
taaseer – the inherent nature of something / the impact or effect of something
fannkaar – artist, writer
qalam – writing instrument

Rough English Translation: There is surely some reason for the lack of an inherent impact in these lines; Either the poet is dishonest, or his pen is.

Hence the literal meaning of this couplet is that the inherent effect (taaseer) of a couple of lines (tehreer) written by a poet should be to mesmerise and awe the reader. If that has not happened, if the poet is honest then his writings must be dishonest.

This couplet has been sung beautifully by the late Jagjit Singh in the classic ghazal ‘na shivale na qalisa na haram jhootein hai…”. A link to the ghazal is below: