Boring and reserved, slip inside for a drink (3,6)
I believe the answer is:
dry sherry
'for a drink' is the definition.
(I've seen this before)
'boring and reserved slip' is the wordplay.
I cannot really see how this works, but
'boring' could be 'dry' (dry can mean dull or boring) and 'dry' is located in the answer.
'slip' could be 'err' (to err is to make an error) and 'err' is found in the answer.
'reserved' could be 'shy' (I've seen this in another clue) and 'shy' is present in the leftover letters.
No letters remain.
This explanation may well be incorrect...
'inside' acts as a link.
(Another definition for dry sherry that I've seen is " drink".)