Time to manage rearing horse (5)
I believe the answer is:
epoch
'time' is the definition.
(I know that epoch is a type of geologic time)
'manage rearing horse' is the wordplay.
'manage' becomes 'cope' (manage can mean to cope or do well).
'rearing' says the letters should be written in reverse (as in a horse rearing up).
'horse' becomes 'H' (abbreviation as in horse power).
'cope' back-to-front is 'epoc'.
'epoc'+'h'='EPOCH'
'to' is the link.
(Other definitions for epoch that I've seen before include "Long period of time" , "Historical age" , "Period of notable events" , "Major division or period of history" , "significant time" .)