It takes ages to manage rearing horse (5)
I believe the answer is:
epoch
'it takes ages' is the definition.
The definition suggests a singular noun which matches the answer.
'manage rearing horse' is the wordplay.
'manage' becomes 'cope' (manage can mean to cope or do well).
'rearing' shows that the letters should be reversed in order (as in a horse rearing up).
'horse' becomes 'h' (synonyms).
'cope' backwards is 'epoc'.
'epoc'+'h'='EPOCH'
'to' is the link.
Can you help me to learn more?
(Other definitions for epoch that I've seen before include "Sure date" , "Historical period" , "Distinctive period of time" , "years" , "Period of notable events" .)