Rather big girl has to reform (7)
I believe the answer is:
largish
'rather big' is the definition.
'largish' can be an answer for 'big' (similar in meaning). I'm not sure about the 'rather' bit.
'girl has to reform' is the wordplay.
'to reform' indicates an anagram (letters in a new form).
'girl'+'has'='girlhas'
'girlhas' with letters rearranged gives 'LARGISH'.
Can you help me to learn more?
(Other definitions for largish that I've seen before include "Somewhat hefty" , "Comfortable shoe" , "Fairly big" , "sizeable proportions" , "Of considerable extent" .)