Device administering drug when mountain climbing rule is broken (9)
I believe the answer is:
nebuliser
'device administering drug' is the definition.
(device which sprays a drug)
'mountain climbing rule is broken' is the wordplay.
'mountain' becomes 'ben' (ben is a kind of mountain).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'broken' indicates an anagram.
'ben' in reverse letter order is 'neb'.
'rule'+'is'='ruleis'
'ruleis' with letters rearranged gives 'uliser'.
'neb'+'uliser'='NEBULISER'
'when' acts as a link.
(Other definitions for nebuliser that I've seen before include "Used to inhale the drug" , "Drug delivery system" , "Spray - blue rinse (anag)" , "dispensed by this" , "Device for producing a fine spray, for inhaling a medicinal drug, say" .)