Allow wild speech supporting head of government (5)
I believe the answer is:
grant
'allow' is the definition.
('grant' can be a synonym of 'allow')
'wild speech supporting head of government' is the wordplay.
'wild speech' becomes 'rant' (I've seen this before**).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'head of' says to take the initial letters.
The first letter of 'government' is 'g'.
'rant' after 'g' is 'GRANT'.
(Other definitions for grant that I've seen before include "nominally associated with Ulysses?" , "Confer" , "Subsidy" , "Bestow; bursary" , "Give freely" .)