Getting on a horse around half nine? Roughly (5)
I believe the answer is:
aging
'getting on' is the definition.
(I've seen this before)
'a horse around half nine? roughly' is the wordplay.
'horse' becomes 'gg' (**).
'around' means one lot of letters goes inside another.
'half' indicates taking half.
'roughly' indicates an anagram.
'nine' halved is 'ni'.
'ni' anagrammed gives 'in'.
'gg' enclosing 'in' is 'ging'.
'a'+'ging'='AGING'
(Other definitions for aging that I've seen before include "Maturing" , "Becoming more grown-up" , "development" , "Getting old" , "in decline" .)