Get too big for unfashionable final part of clothing line (7)
I believe the answer is:
outgrow
'get too big for' is the definition.
(to outgrow something is to get too big for it)
'unfashionable final part of clothing line' is the wordplay.
'unfashionable' becomes 'out' (similar in meaning).
'final part of' says to take the final letters.
'line' becomes 'row' (a row or line of things).
The final letter of 'clothing' is 'g'.
'out'+'g'+'row'='OUTGROW'
(Other definitions for outgrow that I've seen before include "Become too large or too mature for" , "Develop faster or become taller than" , "Develop beyond" , "Get bigger" , "excel in development" .)