Accumulate a source of material when head of state (5)
I believe the answer is:
amass
'accumulate' is the definition.
('amass' can be a synonym of 'accumulate')
'a source of material when head of state' is the wordplay.
'source of' suggests taking the first letters.
'when' becomes 'as'.
'head of' says to take the initial letters.
The initial letter of 'material' is 'm'.
The first letter of 'state' is 's'.
'a'+'m'+'as'+'s'='AMASS'
(Other definitions for amass that I've seen before include "Stockpile" , "Accumulate over time" , "Assemble" , "Accumulate, heap up" , "store" .)