Puny little child, happy at last (5)
I believe the answer is:
weedy
'puny' is the definition.
(I've seen this before)
'little child happy at last' is the wordplay.
'little' becomes 'wee' (similar in meaning).
'at last' says to take the final letters.
The final letters of 'child happy' are 'dy'.
'wee'+'dy'='WEEDY'
(Other definitions for weedy that I've seen before include "Skinny" , "wimpish" , "like an untended flower bed?" , "quite unlike Charles Atlas" , "Underdeveloped" .)