Clever clogs strikes up relationship (7)
I believe the answer is:
smartie
'clever clogs' is the definition.
(both can mean a clever person)
'strikes up relationship' is the wordplay.
'strikes' becomes 'rams' (ram can mean to strike or hit).
'up' is a reversal indicator (in down clue: letters go upwards).
'relationship' becomes 'tie' (tie is a kind of relationship).
'rams' backwards is 'smar'.
'smar'+'tie'='SMARTIE'
(Other definitions for smartie that I've seen before include "Clever clogs" , "Clever dick" .)