An embankment's architect (4)
I believe the answer is:
adam
'architect' is the definition.
(I've seen this before)
'an embankment's' is the wordplay.
'an' becomes 'a'.
'embankment' becomes 'dam' (I've seen this before).
'a'+'dam'='ADAM'
(Other definitions for adam that I've seen before include "Eden man" , "Man on the ceiling of the Sistine Chapel" , "A mad sort of ancestor" , "Robert ..., ... Clayton" , "Biblical first man" .)