Sluggish pitman heading off before time (5)
I believe the answer is:
inert
'sluggish' is the definition.
('inert' can be a synonym of 'sluggish')
'pitman heading off before time' is the wordplay.
'pitman' becomes 'miner' (I've seen this before).
'heading off' means to remove the first letter.
'before' is a charade indicator (letters next to each other).
'time' becomes 't'.
'miner' with its initial letter taken off is 'iner'.
'iner'+'t'='INERT'
(Other definitions for inert that I've seen before include "Motionless or inanimate" , "Like e.g. xenon" , "like nitrogen" , "Chem. non-reactive" , "Incapable of acting, like some gas" .)