A horse - one always turning circles in pain (8)
I believe the answer is:
aggrieve
'pain' is the definition.
(I've seen this before)
'a horse one always turning circles' is the wordplay.
'horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'one' becomes 'i' (Roman numeral).
'always' becomes 'ever' (synonyms).
'turning' says the letters should be written backwards.
'circles' indicates putting letters inside (some letters go round others).
'ever' written backwards gives 'reve'.
'i' inserted within 'reve' is 'rieve'.
'a'+'gg'+'rieve'='AGGRIEVE'
'in' acts as a link.
(Other definitions for aggrieve that I've seen before include "Hurt" , "Cause annoyance to" , "Cause offence or injury to" , "Trouble" , "Do an injustice to" .)