When speaking, pop four pounds in (7)
I believe the answer is:
beetles
'pounds in' is the definition.
(beetle can mean to crush or pound)
'when speaking pop four' is the wordplay.
'when speaking' shows a homophone (sound like).
'pop four' becomes 'beatles' (pop band with four members).
'beatles' sounds like 'BEETLES'.
(Other definitions for beetles that I've seen before include "Bet eels could turn into insects" , "Lets bee turn to insects" , "Four-winged insects" .)