Tourist — one undressing? Not at first (7)
I believe the answer is:
tripper
'tourist' is the definition.
(tripper is a kind of tourist)
'one undressing? not at first' is the wordplay.
'one undressing?' becomes 'stripper'.
'not at first' means to remove the first letter.
'stripper' with its first letter removed is 'TRIPPER'.
(Other definitions for tripper that I've seen before include "Holidaymaker" , "Someone on an excursion" , "person having highly enjoyable experience?" , "at risk of sustaining injury?" , "Tourist" .)