Limit miles covered by old plane perhaps (7)
I believe the answer is:
extreme
'limit' is the definition.
(thesaurus)
'miles covered by old plane perhaps' is the wordplay.
'miles' becomes 'm' (common abbreviation).
'covered by' indicates putting letters inside.
'old' becomes 'ex'.
'plane perhaps' becomes 'tree' (I've seen this in other clues).
'ex'+'tree'='extree'
'm' put within 'extree' is 'EXTREME'.
(Other definitions for extreme that I've seen before include "Severe; radical" , "Utmost; excessive" , "Furthest or highest degree of something" , "Going to great lengths" , "sheer" .)