Drink in during the gym work (4)
I believe the answer is:
pint
'drink' is the definition.
(pint is a unit of liquid volume)
'during the gym work' is the wordplay.
'during' means one lot of letters goes inside another.
'the gym' becomes 'PT' (Physical Training).
'work' becomes 'in' (I can't explain this - if you can you should give a lot more credence to this answer).
'pt' going around 'in' is 'PINT'.
'in' is the link.
Can you help me to learn more?
(Other definitions for pint that I've seen before include "Unit of beer" , "Half a quart" , "Liquid measure" , "Liquid meassure - four gills" , "Imperial measure of capacity" .)