On holiday, queen is volunteer (5)
I believe the answer is:
offer
'volunteer' is the definition.
('offer' can be a synonym of 'volunteer')
'on holiday queen' is the wordplay.
'on holiday' becomes 'off' (I've seen this before).
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'off'+'er'='OFFER'
'is' acts as a link.
(Other definitions for offer that I've seen before include "Tender, volunteer" , "Present for acceptance or rejection" , "Provide; reduced price" , "Chance to buy at reduced price?" , "Volunteer, present" .)