Growth must be seen without popular line (6)
I believe the answer is:
crease
'line' is the definition.
(I know that line can be written as crease)
'growth must be seen without popular' is the wordplay.
'growth' becomes 'increase' (an increase is a growth or rise).
'must be seen without' is a deletion indicator.
'popular' becomes 'in' ('in' can mean trendy).
'increase' with 'in' removed is 'CREASE'.
(Other definitions for crease that I've seen before include "Line caused by folding or pressing, cloth or paper" , "Cricket pitch marking" , "Cricket line; wrinkle" , "Mark made by fold" , "iron this out" .)