Regularly saw firms dip (4)
I believe the answer is:
swim
'dip' is the definition.
(I've seen this in another clue)
'regularly saw firms' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'saw'+'firms'='sawfirms'
The alternating letters of 'sawfirms' are 'SWIM'.
(Other definitions for swim that I've seen before include "Bathe" , "Do the dog paddle, perhaps" , "Move through the water - be dizzy" , "German submarine" , "Do crawl, perhaps" .)