First man heading off for weir? (3)
I believe the answer is:
dam
'weir?' is the definition.
(weir is a kind of dam)
'first man heading off' is the wordplay.
'first man' becomes 'adam' (I've seen this before).
'heading off' means to remove the first letter.
'adam' with its initial letter taken away is 'DAM'.
'for' is the link.
(Other definitions for dam that I've seen before include "Mother of quadruped" , "A mad kind of mum" , "Horse's mother" , "River blockage" , "eg Aswan" .)