Gotten too big for unfashionable dress? Finally, tailor gets brought in (8)
I believe the answer is:
outgrown
'gotten too big for' is the definition.
(I've seen this in another clue)
'unfashionable dress? finally tailor gets brought in' is the wordplay.
'unfashionable' becomes 'out' (similar in meaning).
'dress?' becomes 'gown' (gown is a kind of dress).
'finally' says to take the final letters.
'gets brought in' indicates putting letters inside.
The final letter of 'tailor' is 'r'.
'gown' placed around 'r' is 'grown'.
'out'+'grown'='OUTGROWN'
(Another definition for outgrown that I've seen is " Too big or mature for".)