Young dog eating male gymshoe (4)
I believe the answer is:
pump
'gymshoe' is the definition.
I can't tell whether this definition defines the answer.
'young dog eating male' is the wordplay.
'young dog' becomes 'pup' (short for puppy).
'eating' means one lot of letters goes inside another.
'male' becomes 'm' (common abbreviation).
'pup' enclosing 'm' is 'PUMP'.
Can you help me to learn more?
(Other definitions for pump that I've seen before include "Empty" , "Device for driving air or liquid" , "Compressor, light shoe" , "Machine for driving air or liquid" , "Cross-examine; siphon" .)