Unruly imp with short trousers and onesie (6,4)
I believe the answer is:
romper suit
'onesie' is the definition.
(type of one-piece garment)
'unruly imp with short trousers' is the wordplay.
'unruly' indicates anagramming the letters.
'with' means one lot of letters go next to another.
'short' means to remove the last letter.
'trousers' with its final letter taken away is 'trouser'.
'imp'+'trouser'='imptrouser'
'imptrouser' anagrammed gives 'ROMPER-SUIT'.
'and' is the link.
(Other definitions for romper suit that I've seen before include "Young child's one-piece outer garment" , "Item of clothing" .)