Farthest one leaves exit by mere adjustment (7)
I believe the answer is:
extreme
'farthest' is the definition.
(thesaurus)
'one leaves exit by mere adjustment' is the wordplay.
'one' becomes 'i' (Roman numeral).
'leaves' is a deletion indicator.
'by' is a charade indicator (letters next to each other).
'adjustment' indicates an anagram (the order of letters is adjusted).
'exit' with 'i' taken away is 'ext'.
'mere' is an anagram of 'reme'.
'ext'+'reme'='EXTREME'
(Other definitions for extreme that I've seen before include "Farthest limit" , "Exceptional - radical" , "Sharp" , "Severe; radical" , "Last" .)