Become too big for outing, dropping in line (7)
I believe the answer is:
outgrow
'become too big for' is the definition.
(I've seen this before)
'outing dropping in line' is the wordplay.
'dropping' indicates named letters should be taken away.
'line' becomes 'row' (a row or line of things).
'outing' with 'in' taken away is 'outg'.
'outg'+'row'='OUTGROW'
(Other definitions for outgrow that I've seen before include "Develop beyond" , "Get bigger" , "Become too large or too mature for" , "Become too big for" , "Develop faster or become taller than" .)