All by myself and, oh, need to get jiggy! (3-6)
I believe the answer is:
'all by myself' is the definition.
(one-handed can mean involving only one person)
'and oh need to get jiggy' is the wordplay.
'to get jiggy' indicates an anagram (letters jig or dance around).
'andohneed' anagrammed gives 'ONE-HANDED'.
(Other definitions for one-handed that I've seen before include "very short of staff?", "Not fully getting to grips with things".)