Enemy planned to seize unknown catalyst (6)
I believe the answer is:
enzyme
'catalyst' is the definition.
(enzyme is a kind of catalyst)
'enemy planned to seize unknown' is the wordplay.
'planned' is an anagram indicator.
'to seize' means one lot of letters goes inside another.
'unknown' becomes 'z' (algebra).
'enemy' with letters rearranged gives 'enyme'.
'enyme' placed around 'z' is 'ENZYME'.
(Other definitions for enzyme that I've seen before include "Protein produced by living cells" , "Protein produced by living cell" , "E.g. pepsin" , "Catalytic protein molecule" , "Bodily substance acting as a catalyst" .)