Big-head taking command of the side! (6)
I believe the answer is:
'the side' is the definition.
(thesaurus)
'big-head taking command' is the wordplay.
'big-head' becomes 'b' (head letter of 'big-').
'taking' says to put letters next to each other.
'command' becomes 'order' (order is a kind of command).
'b'+'order'='BORDER'
'of' is the link.
(Other definitions for border that I've seen before include "Line between countries" , "bed?" , "An edgy place" , "Line that indicates a boundry" , "BLINK/BRINK" .)