Minute gee-gee? That is a cat! (6)
I believe the answer is:
moggie
'a cat' is the definition.
(informal British term)
'minute gee-gee? that is' is the wordplay.
'minute' becomes 'mo' (both can mean a short amount of time).
'gee' becomes 'g' (synonyms).
'gee?' becomes 'g' ('g' can be a synonym of 'gee').
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'mo'+'g'+'g'+'ie'='MOGGIE'
(Other definitions for moggie that I've seen before include "Cat (sl.)" , "Domestic cat (slang)" .)