Restrain the first man to lose his head (3)
I believe the answer is:
dam
'restrain' is the definition.
(thesaurus)
'first man to lose his head' is the wordplay.
I cannot quite understand how this works, but
'first' could be 'a' (first letter of the alphabet) and 'a' is present in the answer.
'man' could be 'm' (M is an abbreviation for male or masculine) and 'm' is located in the answer.
A single letter 'd' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'the' is the link.
Can you help me to learn more?
(Other definitions for dam that I've seen before include "Barrier to contain the flow of water" , "Flood barrier" , "eg Aswan" , "Wall restraining water" , "Female parent of an animal, especially livestock" .)