Fix casually? Strictly! (7)
I believe the answer is:
rigidly
'strictly' is the definition.
(thesaurus)
'fix casually?' is the wordplay.
'fix' becomes 'rig' (rigging is a kind of fixing).
'casually?' becomes 'idly'.
'rig'+'idly'='RIGIDLY'
(Other definitions for rigidly that I've seen before include "without bending" , "Stiffly" , "in unyielding grip" .)