Beachwear at first rather lost in the crowd (5)
I believe the answer is:
thong
'beachwear' is the definition.
(I've seen this before)
'first rather lost in the crowd' is the wordplay.
'first rather' becomes 'r' (1st letter of 'rather').
'lost in' is a deletion indicator.
'the crowd' becomes 'throng' (I've seen this before).
'throng' with 'r' taken away is 'THONG'.
'at' is the link.
(Other definitions for thong that I've seen before include "Narrow strip of leather (or other material)" , "Insubstantial underwear" , "Embarrassingly brief underwear" , "Narrow strip of hide used as lash" , "Skimpy underwear" .)