Skimpy swimwear enthralling river crowd (6)
I believe the answer is:
throng
'crowd' is the definition.
(I know that throng is a type of crowd)
'skimpy swimwear enthralling river' is the wordplay.
'skimpy swimwear' becomes 'thong' (skimpy underwear).
'enthralling' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'thong' going around 'r' is 'THRONG'.
(Other definitions for throng that I've seen before include "Press" , "A great crowd of people" , "A large crowd of people" , "Come in great numbers" , "Pack" .)