Curtailment of diver's facilities (3)
I believe the answer is:
loo
'facilities' is the definition.
(I've seen this in other clues)
'curtailment of diver's' is the wordplay.
'curtailment of' means to remove the last letter (to curtail is to cut short).
'diver' becomes 'loon' (synonyms).
'loon' with its last letter taken off is 'LOO'.
(Other definitions for loo that I've seen before include "The little room - card game" , "Card-game" , "Card game; the 'smallest room'" , "Toilet (informal)" , "In which one goes" .)