Accumulate a million as head of state (5)
I believe the answer is:
amass
'accumulate' is the definition.
('amass' can be a synonym of 'accumulate')
'a million as head of state' is the wordplay.
'million' becomes 'm' (abbreviation).
'head of' suggests taking the first letters.
The first letter of 'state' is 's'.
'a'+'m'+'as'+'s'='AMASS'
(Other definitions for amass that I've seen before include "Stock up on" , "Accrue, heap up" , "Sodium chloride" , "Assemble" , "Be a compiler" .)