No match official accepted without a uniform (7)
I believe the answer is:
refusal
'no' is the definition.
(refusing is saying no)
'match official accepted without a uniform' is the wordplay.
'match official' becomes 'ref' (referee).
'accepted' becomes 'sal' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'a uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'ref'+'sal'='refsal'
'refsal' going around 'u' is 'REFUSAL'.
Can you help me to learn more?
(Other definitions for refusal that I've seen before include "Failure to accept" , "Negative response" , "Chance to decline" , "Act of a horse stopping short at a jump" , "Indication of unwillingness (to do something)" .)