Promise singular sport (5)
I believe the answer is:
swear
'promise' is the definition.
(thesaurus)
'singular sport' is the wordplay.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'sport' becomes 'wear' (I've seen this before).
's'+'wear'='SWEAR'
(Other definitions for swear that I've seen before include "Promise on oath" , "Assert" , "Utter obscenities or take an oath" , "Use indecent or blasphemous language" , "Avow" .)