Slow at thinking, displays a bit of power (4)
I believe the answer is:
watt
'a bit of power' is the definition.
(I've seen this in another clue)
'slow at thinking displays' is the wordplay.
'displays' says the answer is hidden in the clue (part of the clue displays the answer).
'WATT' is hidden within 'slow at thinking'.
(Other definitions for watt that I've seen before include "Scot. engineer; unit of power" , "bit of power" , "Unit of power -- Scottish engineer, d. 1819" , "Worker in steam" , "Man of note" .)