Occasionally fond of an oddball (2,3,3)
I believe the answer is:
on and off
'occasionally' is the definition.
(I've seen this before)
'fond of an oddball' is the wordplay.
'oddball' indicates an anagram.
'fond'+'of'+'an'='fondofan'
'fondofan' with letters rearranged gives 'ON-AND-OFF'.
(Other definitions for on and off that I've seen before include "Occasionally" , "From time to time, not continuously" , "spasmodic" , "periodically" , "the two sides at Old Trafford?" .)