Appear to relish swallowing a double (9)
I believe the answer is:
lookalike
'double' is the definition.
(I've seen this before)
'appear to relish swallowing a' is the wordplay.
'appear' becomes 'look' ('look' can be a synonym of 'appear'**).
'to relish' becomes 'like' (I've seen this before).
'swallowing' indicates putting letters inside.
'look'+'like'='looklike'
'looklike' going around 'a' is 'LOOKALIKE'.
(Other definitions for lookalike that I've seen before include "twin?" , "Copy" , "Doppelganger" , "Double" .)