Front line in tie (5)
I believe the answer is:
blind
'front' is the definition.
(thesaurus)
'line in tie' is the wordplay.
'line' becomes 'l'.
'in' is an insertion indicator.
'tie' becomes 'bind' ('bind' can be a synonym of 'tie'**).
'l' placed inside 'bind' is 'BLIND'.
(Other definitions for blind that I've seen before include "Unobserving" , "cover-up" , "In the dark" , "Drive very fast" , "without a way out" .)