One may win it by a head (4)
I believe the answer is:
toss
'a head' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'one may win it by' is the wordplay.
'one may win' becomes 'oss' (I can't explain this - if you can you should give a lot more credence to this answer).
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'by' is a charade indicator (letters next to each other).
'oss' after 't' is 'TOSS'.
Can you help me to learn more?
(Other definitions for toss that I've seen before include "Pitch" , "Flip in the air, sald or coin" , "Lob" , "ball" , "Throw (coin) so as to choose" .)