Engineers a pierhead, we gather (4)
I believe the answer is:
reap
'gather' is the definition.
(reaping is a kind of gathering)
'engineers a pierhead' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'pierhead' becomes 'p' (head letter of 'pier').
're'+'a'+'p'='REAP'
'we' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for reap that I've seen before include "Cut or gather (a harvest)" , "Use a scythe" , "'As you sow, so shall you . . . .'" , "Cut and gather (crop)" , "Gather 23dn" .)