Obstruct prurient fools heading for threesome (9)
I believe the answer is:
interrupt
'obstruct' is the definition.
(thesaurus)
'prurient fools heading for threesome' is the wordplay.
'fools' is an anagram indicator (I've seen this in other clues).
'heading for' suggests taking the first letters.
The initial letter of 'threesome' is 't'.
'prurient' anagrammed gives 'interrup'.
'interrup'+'t'='INTERRUPT'
(Other definitions for interrupt that I've seen before include "Break the continuity of" , "middle of ashtray after burning slip gets put out" .)