So grown-up it's hard to believe! (4)
I believe the answer is:
tall
'hard to believe' is the definition.
(as in a tall tale)
'so grown-up it's' is the wordplay.
I cannot quite understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
The remaining letters 'all' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tall that I've seen before include "Lofty; unlikely" , "Bigger than average" , "Vertically large" , "that's hard to swallow" , "Of great stature" .)