In which room-mates may swot (4)
I believe the answer is:
twos
'in which room-mates' is the definition.
Although both the answer and 'in' are nouns, they are not in the same form of the noun.
'may swot' is the wordplay.
'may' is an anagram indicator (I've seen 'might' mean this (I've seen this in other clues)).
'swot' anagrammed gives 'TWOS'.
Can you help me to learn more?
(Other definitions for twos that I've seen before include "Pairs" , "Couples" , "Deuces" .)