Developer surrounding unit with buttress (7)
I believe the answer is:
pioneer
'developer' is the definition.
(I have seen 'Original developer' mean 'pioneer' so perhaps 'developer' could also mean 'pioneer')
'surrounding unit with buttress' is the wordplay.
'surrounding' means one lot of letters goes inside another.
'unit with' becomes 'one' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'buttress' becomes 'pier' (I've seen this before).
'one' put into 'pier' is 'PIONEER'.
(Other definitions for pioneer that I've seen before include "spacecraft" , "Innovator; first settler" , "Colonist" , "Research leader" , "Frontiersman" .)