Time left, having unknown score (6)
I believe the answer is:
twenty
'score' is the definition.
(synonyms)
'time left having unknown' is the wordplay.
'time' becomes 't' (abbreviation).
'left' becomes 'went' (left can mean went away).
'having' means one lot of letters go next to another.
'unknown' becomes 'y' (algebra).
't'+'went'+'y'='TWENTY'
(Other definitions for twenty that I've seen before include "made by Q on a light blue square" , "Figure" , "Score, XX" , "No. between 5 and 1 on dartboard" , "John B Keane wrote about many young men of this age" .)