Race in trunks for quantities of drink (7)
I believe the answer is:
bottles
'quantities of drink' is the definition.
'bottles' can be an answer for 'quantities' (bottle is a kind of quantity). I am not sure about the 'of drink' bit.
'race in trunks' is the wordplay.
'race' becomes 'tt'.
'in' indicates putting letters inside.
'trunks' becomes 'boles' (I can't explain this - if you can you should give a lot more credence to this answer).
'tt' inserted within 'boles' is 'BOTTLES'.
'for' is the link.
Can you help me to learn more?
(Another definition for bottles that I've seen is " Containers for liquid".)