Winning's apt when holding high card (7)
I believe the answer is:
likable
'winning's' is the definition.
(thesaurus)
'apt when holding high card' is the wordplay.
'apt' becomes 'liable' (synonyms).
'when holding' is an insertion indicator.
'high card' becomes 'k' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'liable' enclosing 'k' is 'LIKABLE'.
Can you help me to learn more?
(Other definitions for likable that I've seen before include "Charming" , "attractive" , "Pleasant -- genial" , "Sympathetic" , "Amiable" .)