One skilled amateur trains frantically (7)
I believe the answer is:
artisan
'one skilled' is the definition.
(skilled craftsperson)
'amateur trains frantically' is the wordplay.
'amateur' becomes 'a' ().
'frantically' indicates anagramming the letters.
'trains' with letters rearranged gives 'rtisan'.
'a'+'rtisan'='ARTISAN'
(Other definitions for artisan that I've seen before include "Mechanic" , "Skilled worker at craft or trade" , "Handy worker" , "Sinatra (anag) -- skilled manual worker" , "craftsman of his trade" .)