Scheme of the port authority at central Greenwich (4)
I believe the answer is:
plan
'scheme' is the definition.
(thesaurus)
'the port authority at central greenwich' is the wordplay.
'the port authority' becomes 'pla' (Port of London Authority).
'at' means one lot of letters go next to another.
'central' indicates the central letters.
The central letter of 'greenwich' is 'n'.
'pla'+'n'='PLAN'
'of' acts as a link.
(Other definitions for plan that I've seen before include "Arrangement" , "Scheme, design" , "Devise a way to achieve" , "Diagram; design" , "'Scheme, strategy (4)'" .)