Periodically saw firms float (4)
I believe the answer is:
swim
'float' is the definition.
(synonyms)
'periodically saw firms' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'saw'+'firms'='sawfirms'
The alternate letters of 'sawfirms' are 'SWIM'.
(Other definitions for swim that I've seen before include "Move through the water - be dizzy" , "Ooze out; show lots of (confidence)" , "progress through water" , "Do the butterfly, perhaps?" , "Crawl, e.g." .)