Peer with sidearm at intervals previously (7)
I believe the answer is:
earlier
'previously' is the definition.
(synonyms)
'peer with sidearm at intervals' is the wordplay.
'peer' becomes 'earl' (earl is a kind of peer).
'with' means one lot of letters go next to another.
'at intervals' means one should take alternating letters (letters taken at regular intervals).
The alternating letters of 'sidearm' are 'ier'.
'earl'+'ier'='EARLIER'
(Other definitions for earlier that I've seen before include "of a bygone age?" , "Sooner in time" , "in days of yore?" , "Formerly" , "At a previous time" .)