Make small improvements to a motorway before completion (5)
I believe the answer is:
amend
'make small improvements to' is the definition.
(I've seen this before)
'a motorway before completion' is the wordplay.
'motorway' becomes 'm'.
'before' says to put letters next to each other.
'completion' becomes 'end' (I've seen this before).
'a'+'m'+'end'='AMEND'
(Other definitions for amend that I've seen before include "Revise" , "Modify - edit" , "Alter in detail" , "Make minor improvements to (something)" , "Correct error, make minor alteration as to writing" .)