Stick chlorine in bottom of jug (5)
I believe the answer is:
cling
'stick' is the definition.
('cling' can be a synonym of 'stick')
'chlorine in bottom of jug' is the wordplay.
'chlorine' becomes 'cl' (chemical symbol).
'bottom of' indicates one should take the final letters.
The last letter of 'jug' is 'g'.
'cl'+'in'+'g'='CLING'
(Other definitions for cling that I've seen before include "have emotional attachment?" , "Adhere, hold tight" , "fast" , "Keep hold" , "Grasp urgently" .)