Develop popular line (8)
I believe the answer is:
increase
'develop' is the definition.
(thesaurus)
'popular line' is the wordplay.
'popular' becomes 'in'.
'line' becomes 'crease' ('crease' can be a synonym of 'line'**).
'in'+'crease'='INCREASE'
(Other definitions for increase that I've seen before include "Increment" , "Add, make bigger" , "Make (amount) greater" , "Get bigger" , "rise in wages, perhaps" .)