Become too large or too mature for (7)
I believe the answer is:
outgrow
'become too large or too mature for' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for outgrow that I've seen before include "Leave (something) behind as one matures" , "Become too big for" , "excel in development" , "Develop faster or become taller than" , "Get too big for" .)