Form of jailbird facing spell (7)
I believe the answer is:
contour
'form' is the definition.
('contour' can be a synonym of 'form')
'jailbird facing spell' is the wordplay.
'jailbird' becomes 'con' (I've seen this in other clues).
'facing' means one lot of letters go next to another.
'spell' becomes 'tour' ('tour' can be a synonym of 'spell').
'con'+'tour'='CONTOUR'
'of' is the link.
(Other definitions for contour that I've seen before include "Outline, profile" , "Feature on map" , "Silhouette" , "Line on map indicating height" , "Outline, shape" .)