Grow in line (8)
I believe the answer is:
increase
'grow' is the definition.
(thesaurus)
'in line' is the wordplay.
'line' becomes 'crease' ('crease' can be a synonym of 'line'**).
'in'+'crease'='INCREASE'
(Other definitions for increase that I've seen before include "Addition, growth" , "Augment or enlarge (8)." , "Breed - nicer sea (anag)" , "Become bigger or greater in amount" , "Increment" .)