'Grow up pal', she said, 'you are in' (6)
I believe the answer is:
mature
'grow up pal' is the definition.
'mature' can be an answer for 'grow' (synonyms). I'm unsure of the 'up pal' bit.
'she said you are in' is the wordplay.
'she said' becomes 'mate' (I can't justify this - if you can you should give a lot more credence to this answer).
'you are' becomes 'ur' (textspeak abbreviation).
'in' is an insertion indicator.
'mate' placed around 'ur' is 'MATURE'.
Can you help me to learn more?
(Other definitions for mature that I've seen before include "Grown-up" , "Grow older" , "due to be paid" , "Fully-developed" , "Age" .)