Sentry in nude condition is vulnerable (9)
I believe the answer is:
unguarded
'vulnerable' is the definition.
(similar in meaning)
'sentry in nude condition' is the wordplay.
'sentry' becomes 'guard' (I've seen this in another clue).
'in' indicates putting letters inside.
'condition' indicates an anagram (I've seen 'conditions' mean this (letters must be in a new condition)).
'nude' anagrammed gives 'uned'.
'guard' placed into 'uned' is 'UNGUARDED'.
'is' is the link.
(Other definitions for unguarded that I've seen before include "Imprudent" , "Open to attack" , "like man liable to be taken on board" , "Not protected" .)