Nuts can fit, actually (2,4)
I believe the answer is:
in fact
'actually' is the definition.
(synonyms)
'nuts can fit' is the wordplay.
'nuts' indicates an anagram (nuts can mean crazy).
'can'+'fit'='canfit'
'canfit' is an anagram of 'IN FACT'.
(Other definitions for in fact that I've seen before include "really!" , "Actually" , "'In reality (2,4)'" .)