Add to popular line (8)
I believe the answer is:
increase
'add to' is the definition.
(I've seen this before)
'popular line' is the wordplay.
'popular' becomes 'in' (something that's 'in' is fashionable).
'line' becomes 'crease' (synonyms).
'in'+'crease'='INCREASE'
(Other definitions for increase that I've seen before include "Grow in size" , "Profit" , "Make or grow larger" , "Grow bigger" , "Grow greater" .)