Always enthralled by cleavage, but is respectful (8)
I believe the answer is:
reverent
'is respectful' is the definition.
(similar in meaning)
'always enthralled by cleavage' is the wordplay.
'always' becomes 'ever' (synonyms).
'enthralled by' indicates putting letters inside ('enthral' can mean 'capture').
'cleavage' becomes 'rent' (both can mean a tear or split in something).
'ever' put inside 'rent' is 'REVERENT'.
'but' is the link.
(Other definitions for reverent that I've seen before include "Deeply respectful" , "showing much respect" , "worshipping" , "humble" , "Showing great respect" .)