In which to fish on a cold morning? (3)
I believe the answer is:
cam
'in which to fish' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'on a cold morning?' is the wordplay.
'on' means one lot of letters go next to another.
'a cold' becomes 'c' (abbreviation, eg on taps).
'morning?' becomes 'am' (as in 12:00 AM).
'c'+'am' is 'CAM'.
Can you help me to learn more?
(Other definitions for cam that I've seen before include "Rotating part in machine; river" , "Irregular projection" , "It controls the motion of engine cylinder valves" , "Shaft on a rotating part in machinery" , "running through university city?" .)