Cat returned with her first insect (4)
I believe the answer is:
moth
'insect' is the definition.
(moth is a kind of insect)
'cat returned with her first' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'returned' says the letters should be written in reverse.
'with' means one lot of letters go next to another.
'her first' becomes 'h' (1st letter of 'her').
'tom' in reverse letter order is 'mot'.
'mot'+'h'='MOTH'
(Other definitions for moth that I've seen before include "Cloth-eating insect" , "One of 5 [FAIRIES]" , "Fairy like 13" , "Night-flier" , "Insect drawn to flame" .)