Enemy managed to capture unknown catalyst (6)
I believe the answer is:
enzyme
'catalyst' is the definition.
(enzyme is a kind of catalyst)
'enemy managed to capture unknown' is the wordplay.
'managed' indicates an anagram.
'to capture' is an insertion indicator.
'unknown' becomes 'z' (algebra).
'enemy' with letters rearranged gives 'enyme'.
'enyme' going around 'z' is 'ENZYME'.
(Other definitions for enzyme that I've seen before include "One causing a reaction" , "E.g. pepsin" , "Bodily substance acting as a catalyst" , "Biological catalyst" , "Cell-produced protein" .)