A golfer can so very easily do it in! (3)
I believe the answer is:
tap
'in' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'a golfer can so very easily do it' is the wordplay.
I cannot really understand how this works, but
'a' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
A single letter 'p' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tap that I've seen before include "water supply" , "Dancing" , "Secret (phone) intercept" , "that's less hard than 11 [BASH] or 22 [BATTER]" , "It's faucet in the US" .)