Popular true, but not precise (7)
I believe the answer is:
inexact
'not precise' is the definition.
(I've seen this before)
'popular true' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'true' becomes 'exact' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in'+'exact'='INEXACT'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for inexact that I've seen before include "Not entirely accurate" , "Rough" , "Fuzzy, muddled" , "Not quite accurate" , "Nice tax (anag) - approximate" .)