Volunteers and politician forming compact (4)
I believe the answer is:
tamp
'compact' is the definition.
(I know this)
'volunteers and politician' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'and' means one lot of letters go next to another.
'politician' becomes 'MP' (Member of Parliament).
'ta'+'mp'='TAMP'
'forming' is the link.
(Other definitions for tamp that I've seen before include "Push down tobacco" , "Compact" , "Pack or ram down tightly" , "Press down tightly" , "Pack, force down" .)