Rain heavily on squad, say (4)
I believe the answer is:
teem
'rain heavily' is the definition.
(I've seen this before)
'squad say' is the wordplay.
'squad' becomes 'team' (synonyms).
'say' shows a homophone (sound like).
'team' is a homophone of 'TEEM'.
'on' is the link.
(Other definitions for teem that I've seen before include "stream" , "Flow copiously" , "Pullulate" , "Bucket down" , "(Of rain) fall heavily" .)