Love less, assuming nothing sinister (7)
I believe the answer is:
ominous
'sinister' is the definition.
(I know that sinister can be written as ominous)
'love less assuming nothing' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'less' becomes 'minus' (I've seen this before).
'assuming' is an insertion indicator (inserted letters are taken in or assumed).
'nothing' becomes 'o' (O resembles 0).
'minus' enclosing 'o' is 'minous'.
'o'+'minous'='OMINOUS'
(Other definitions for ominous that I've seen before include "Threatening, portending tragedy" , "Portending evil" , "Minatory, trouble coming" , "Threatening, looks bad" , "Foreboding, menacing" .)