Relations stuck in loo, turning key (5)
I believe the answer is:
vital
'key' is the definition.
(I know that key can be written as vital)
'relations stuck in loo turning' is the wordplay.
'relations' becomes 'it' (both can mean sex).
'stuck in' indicates putting letters inside.
'loo' becomes 'lav' (lavatory).
'turning' says the letters should be written in reverse.
'lav' reversed gives 'val'.
'it' inserted inside 'val' is 'VITAL'.
(Other definitions for vital that I've seen before include "Spirited" , "Energetic - displaying life" , "Are such statistics" , "Crucial" , "Necessary for life" .)