Head of state tending to alarm (5)
I believe the answer is:
scare
'to alarm' is the definition.
(I've seen this in another clue)
'head of state tending' is the wordplay.
'head of' suggests taking the first letters.
'tending' becomes 'care' (synonyms).
The first letter of 'state' is 's'.
's'+'care'='SCARE'
(Other definitions for scare that I've seen before include "Cause fright, alarm" , "Health worry" , "Startle" , "Frighten or intimidate" , "Panic" .)