Second-hand German missile decayed (9)
I believe the answer is:
vicarious
'second-hand' is the definition.
(I've seen this before)
'german missile decayed' is the wordplay.
'german missile' becomes 'vi' (V-1, German missile in the Second World War).
'decayed' becomes 'carious' (synonyms).
'vi'+'carious'='VICARIOUS'
(Other definitions for vicarious that I've seen before include "Experienced at secondhand" , "Acting for another" , "Indirect" , "second hand" , "Acting for others" .)