Cat’s up, before one round it’s withdrawn (8)
I believe the answer is:
taciturn
'withdrawn' is the definition.
(I've seen this before)
'cat's up before one round' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'before' says to put letters next to each other.
'one' becomes 'i' (roman numeral).
'round' becomes 'turn' (synonyms).
'cat' reversed gives 'tac'.
'tac'+'i'+'turn'='TACITURN'
'it's' acts as a link.
(Other definitions for taciturn that I've seen before include "Far from talkative" , "Reticent" , "Quiet and uncommunicative" , "Reserved and uncommunicative" , "Uncommunicative, of few words" .)