Oddly keen to block friendly state, one involved in assembly (5,3)
I believe the answer is:
allen key
'one involved in assembly' is the definition.
(an allen key might be used to assemble something)
'oddly keen to block friendly state' is the wordplay.
'oddly' indicates an anagram.
'to block' indicates putting letters inside (block can mean to plug a gap).
'friendly state' becomes 'ally' (I've seen this in another clue).
'keen' anagrammed gives 'enke'.
'enke' placed inside 'ally' is 'ALLEN KEY'.
(Other definitions for allen key that I've seen before include "Hexagonal spanner for certain screws" , "Form of spanner" .)