Volunteers seen with monarch stealing (6)
I believe the answer is:
taking
'stealing' is the definition.
(I've seen this before)
'volunteers seen with monarch' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'seen with' is a charade indicator (letters next to each other).
'monarch' becomes 'king' (king is a kind of monarch).
'ta'+'king'='TAKING'
(Other definitions for taking that I've seen before include "Receiving; renting" , "infectious" , "acceptance" , "Charming, removing" , "Removal" .)