Stable and occasionally strong anaesthetic (8)
I believe the answer is:
together
'stable' is the definition.
(I've seen this before)
'occasionally strong anaesthetic' is the wordplay.
'occasionally' indicates alternate letters.
'anaesthetic' becomes 'ether' (ether is a kind of anaesthetic).
The alternating letters of 'strong' are 'tog'.
'tog'+'ether'='TOGETHER'
'and' acts as a link.
(Other definitions for together that I've seen before include "In combination" , "in league" , "United; in agreement" , "Collected" , "Not apart" .)