Periodically they arm squad (4)
I believe the answer is:
team
'squad' is the definition.
('team' can be a synonym of 'squad')
'periodically they arm' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
'they'+'arm'='theyarm'
The alternate letters of 'theyarm' are 'TEAM'.
(Other definitions for team that I've seen before include "Crew" , "eleven?" , "Group or squad" , "See 22" , "Group pulling together" .)