A moggy about to steal tumbler (7)
I believe the answer is:
acrobat
'tumbler' is the definition.
(synonyms)
'a moggy about to steal' is the wordplay.
'moggy' becomes 'cat' (I've seen this before).
'about' means one lot of letters goes inside another.
'to steal' becomes 'rob' (robbing is a kind of stealing).
'cat' enclosing 'rob' is 'crobat'.
'a'+'crobat'='ACROBAT'
(Other definitions for acrobat that I've seen before include "Athletic Circus Performer" , "Circus athlete" , "Gymnastic entertainer" , "Circus gymnast" , "Tumbler" .)