I say you have entered first night stark naked and with moustache and beard (8)
I believe the answer is:
unshaven
'with moustache and beard' is the definition.
I can't tell whether this defines the answer.
'i say you have entered first night stark naked' is the wordplay.
'i say you' becomes 'u' (I can't justify this - if you can you should believe this answer much more).
'entered' is an insertion indicator.
'first' indicates taking the first letters.
The first letters of 'night stark naked' is 'nsn'.
'have' placed within 'nsn' is 'nshaven'.
'u'+'nshaven'='UNSHAVEN'
'and' is the link.
Can you help me to learn more?
(Other definitions for unshaven that I've seen before include "Bearded characters" , "Not having removed incipient beard" , "whiskers to deal with?" , "Showing facial stubble" , "Stubbly" .)