Time on top of mature bull (5)
I believe the answer is:
tripe
'bull' is the definition.
(thesaurus)
'time on top of mature' is the wordplay.
'on top of' indicates taking the first letters (I've seen 'on top' mean this).
'mature' becomes 'ripe' (similar in meaning).
The initial letter of 'time' is 't'.
't'+'ripe'='TRIPE'
(Other definitions for tripe that I've seen before include "Swallowed" , "Rubbish - it's ox stomach linings" , "Pants" , "which a freegan might eat?" , "Edible lining of cow's stomach - rubbish" .)