Safety device in rocket, mode set to malfunction (5,8)
I believe the answer is:
smoke detector
'safety device' is the definition.
(I've seen this before)
'rocket mode set to malfunction' is the wordplay.
'to malfunction' is an anagram indicator.
'rocket'+'mode'+'set'='rocketmodeset'
'rocketmodeset' anagrammed gives 'SMOKE DETECTOR'.
'in' acts as a link.
(Other definitions for smoke detector that I've seen before include "Kitchen safety device" , "Fire safety device" , "Home safety device" , "Indoor safety device" , "It warns of fire" .)