Prone to drink plonk, taking cap off (6)
I believe the answer is:
supine
'prone' is the definition.
(thesaurus)
'drink plonk taking cap off' is the wordplay.
'drink' becomes 'sup' (to sup is to drink).
'plonk' becomes 'wine' (I've seen this before).
'taking cap off' means to remove the first letter (the top letter is the cap of the word).
'wine' with its first letter removed is 'ine'.
'sup'+'ine'='SUPINE'
'to' is the link.
(Other definitions for supine that I've seen before include "Unresisting" , "offering no resistance" , "looking up" , "Recumbent" , "Lying face upward" .)