Trainee always has jemmy (5)
I believe the answer is:
lever
'jemmy' is the definition.
(I've seen this before)
'trainee always' is the wordplay.
'trainee' becomes 'l'.
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'l'+'ever'='LEVER'
'has' acts as a link.
(Other definitions for lever that I've seen before include "Means of exerting force" , "Bar operating machinery" , "Crowbar" , "Bar turning on a fulcrum" , "Bar for prising something" .)