Skill is a necessity principally for mechanic (7)
I believe the answer is:
artisan
'mechanic' is the definition.
(thesaurus)
'skill is a necessity principally' is the wordplay.
'skill' becomes 'art' (I've seen this before).
'principally' says to take the initial letters.
The first letter of 'necessity' is 'n'.
'art'+'is'+'a'+'n'='ARTISAN'
'for' is the link.
(Other definitions for artisan that I've seen before include "Journeyman" , "craftsman of his trade" , "Crafty sort" , "crafty person?" , "Handy worker" .)