Get too big for blooming argument about key (7)
I believe the answer is:
outgrow
'get too big for' is the definition.
(to outgrow something is to get too big for it)
'blooming argument about key' is the wordplay.
'blooming' becomes 'out' (plants which have bloomed are 'out').
'argument' becomes 'row' (to row is to argue).
'about' is a charade indicator (letters next to each other).
'key' becomes 'g' (musical key).
'row' after 'g' is 'grow'.
'out'+'grow'='OUTGROW'
(Other definitions for outgrow that I've seen before include "Develop beyond" , "Become too large or too mature for" , "Become too big for" , "Develop faster or become taller than" , "Leave (something) behind as one matures" .)