Mimic in part of breastwork (7)
I believe the answer is:
parapet
'breastwork' is the definition.
('parapet' can be a synonym of 'breastwork')
'mimic in part' is the wordplay.
'mimic' becomes 'ape' (I've seen this before).
'in' indicates putting letters inside.
'ape' put within 'part' is 'PARAPET'.
'of' acts as a link.
(Other definitions for parapet that I've seen before include "form of defence" , "Balcony railing" , "wall used for protection" , "works" , "Low wall along the edge of a balcony" .)