Beset by mire, team’s first half’s quiet (5)
I believe the answer is:
muted
'quiet' is the definition.
(similar in meaning)
'beset by mire team's first half's' is the wordplay.
'beset by' indicates putting letters inside ('beset' could historically mean 'cover').
'mire' becomes 'mud' (synonyms).
'first half' indicates taking half.
'team' cut in half is 'te'.
'mud' enclosing 'te' is 'MUTED'.
(Other definitions for muted that I've seen before include "Not loud" , "Muffled or silent" , "(Of colour) subdued" , "Hushed, dulled or deadened" , "Piano" .)