Moggy in trolley right to leave (3)
I believe the answer is:
cat
'moggy' is the definition.
(I've seen this before)
'trolley right to leave' is the wordplay.
'trolley' becomes 'cart' (I've seen this in another clue).
'right' becomes 'r' (common abbreviation).
'to leave' is a deletion indicator.
'cart' with 'r' removed is 'CAT'.
'in' is the link.
(Other definitions for cat that I've seen before include "A tom" , "Pussy" , "Eg, tiger, lion" , "Malicious gossip" , "Common pet" .)