Renounce Circle Line train (8)
I believe the answer is:
'renounce' is the definition.
(I know that disclaim is a more specific form of the action renounce)
'circle line train' is the wordplay.
'circle' becomes 'disc' (both can mean a round object).
'line' becomes 'l' (used when specifying particular lines from a poem).
'train' becomes 'aim' (synonyms).
(Other definitions for disclaim that I've seen before include "Renounce" , "Deny or relinquish" , "Disown or deny, maybe responsibility" , "Forswear" , "Deny, refuse to acknowledge" .)