Girl has developed, becoming quite big (7)
I believe the answer is:
largish
'quite big' is the definition.
'largish' can be an answer for 'big' (similar in meaning). I am unsure of the 'quite' bit.
'girl has developed' is the wordplay.
'developed' is an anagram indicator.
'girl'+'has'='girlhas'
'girlhas' with letters rearranged gives 'LARGISH'.
'becoming' acts as a link.
Can you help me to learn more?
(Other definitions for largish that I've seen before include "sizeable proportions" , "Fairly big" , "quite generous" , "Of considerable extent" , "Somewhat hefty" .)