Judge ties up partner (8)
I believe the answer is:
estimate
'judge' is the definition.
(synonyms)
'ties up partner' is the wordplay.
'up' indicates an anagram (letters get chucked up).
'partner' becomes 'mate' (synonyms).
'ties' anagrammed gives 'esti'.
'esti'+'mate'='ESTIMATE'
(Other definitions for estimate that I've seen before include "Reckon value" , "Statement giving the likely cost of a job" , "Make approximate judgement of size or amount" , "'Judge, calculate (8)'" , "Judgement of approximate value or size" .)