Skilled worker needed when a train's derailed (7)
I believe the answer is:
artisan
'skilled worker' is the definition.
(an artisan is a skilled craftsperson)
'a train's derailed' is the wordplay.
'derailed' is an anagram indicator (I've seen this in other clues).
'trains' anagrammed gives 'rtisan'.
'a'+'rtisan'='ARTISAN'
'needed when' is the link.
(Other definitions for artisan that I've seen before include "Tsarina (anag.)" , "Skilled worker or craftsman" , "man with skill" , "Sinatra (anag) -- skilled manual worker" , "Journeyman" .)