Unforgettable ace taking part in The Chase (8)
I believe the answer is:
haunting
'unforgettable' is the definition.
(similar in meaning)
'ace taking part in the chase' is the wordplay.
'ace' becomes 'a' (abbreviation).
'taking part in' means one lot of letters goes inside another.
'the chase' becomes 'hunting' (I've seen this before).
'a' going within 'hunting' is 'HAUNTING'.
(Other definitions for haunting that I've seen before include "Frequenting a place, as a ghost might" , "Ghost visitation on ninth Aug" , "difficult to forget" , "A show of spirit" , "Visiting as a ghost" .)