Mum true form of grown-up (6)
I believe the answer is:
mature
'grown-up' is the definition.
(I've seen this before)
'mum true form' is the wordplay.
'mum' becomes 'ma' (both can mean mother).
'form' indicates an anagram (another form of the letters).
'true' is an anagram of 'ture'.
'ma'+'ture'='MATURE'
'of' acts as a link.
(Other definitions for mature that I've seen before include "Age" , "Grow older" , "Ripe or fully grown" , "reach the end of term" , "sensible" .)