Drilled when at a loose end (5)
I believe the answer is:
bored
This clue is a double definition.
'drilled' is the first definition.
(I've seen this before)
'at a loose end' is the second definition.
'when' acts as a link.
(Other definitions for bored that I've seen before include "Filled with ennui" , "Palled on" , "Wearied by tedium" , "World-weary" , "Filled with tedium" .)