Security for instance covering most of fair (6)
I believe the answer is:
safety
'security' is the definition.
(thesaurus)
'for instance covering most of fair' is the wordplay.
'for instance' becomes 'say'.
'covering' is an insertion indicator.
'most of' means to remove the last letter.
'fair' becomes 'fete' (I've seen this before).
'fete' with its last letter removed is 'fet'.
'say' placed around 'fet' is 'SAFETY'.
(Other definitions for safety that I've seen before include "The condition of being free from danger" , "Freedom from risk or danger, or an acrobat's net" , "Welfare" , "Sort of catch" , "The condition of being protected from danger" .)