Met up outside hotel? Not us (4)
I believe the answer is:
them
'not us' is the definition.
(I've seen this in another clue)
'met up outside hotel?' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'outside' means one lot of letters goes inside another.
'hotel?' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'met' in reverse letter order is 'tem'.
'tem' placed around 'h' is 'THEM'.
(Other definitions for them that I've seen before include "Others" , "Certainly not us, the other lot" , "Those people (as object)" , "people objectively" , "Let -- eat cake" .)