Spontaneous and not subject to mediation in advance. About time! (14)

'spontaneous' is the definition.
(thesaurus)

'not subject to mediation in advance about time' is the wordplay.
I cannot quite see how this works, but
'not' could be 'un' (prefix meaning 'not') and 'un' is present in the answer.
'about' could be 're' ('re' means regarding or about) and 're' is located in the answer.
'time' could be 'pm' and 'pm' is located in the remaining letters.
This explanation may well be incorrect...

'and' acts as a link.

Can you help me to learn more?