Lots of people run in underpants (6)
I believe the answer is:
throng
'lots of people' is the definition.
(term for a crowd or group)
'run in underpants' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'in' is an insertion indicator.
'underpants' becomes 'thong' (thong is a type of underwear).
'r' inserted within 'thong' is 'THRONG'.
(Other definitions for throng that I've seen before include "Large gathering of people" , "Large packed crowd" , "Large densely-packed crowd" , "Come in great numbers" , "A crowded mass of people" .)