Spike Milligan ultimately trapped by 'Spike' (5)
I believe the answer is:
lance
'spike' is the definition.
(thesaurus)
'milligan ultimately trapped by spike' is the wordplay.
'ultimately' indicates one should take the final letters.
'trapped by' is an insertion indicator.
'spike' becomes 'lace' ('lace' can be a synonym of 'spike').
The last letter of 'milligan' is 'n'.
'n' inserted within 'lace' is 'LANCE'.
(Other definitions for lance that I've seen before include "A horse soldier's spear" , "Jousting weapon" , "Knightly weapon" , "Weapon of knights of old" , "Cut open" .)