Spike Milligan's finale: to be ill (4)
I believe the answer is:
nail
'spike' is the definition.
(I've seen this before)
'milligan's finale to be ill' is the wordplay.
'finale' says to take the final letters.
'to' means one lot of letters go next to another.
'be ill' becomes 'ail' (synonyms).
The final letter of 'milligan' is 'n'.
'n'+'ail'='NAIL'
(Other definitions for nail that I've seen before include "Hammered pin" , "Thin pointed fastener" , "Claw" , "Metal fastener" , "Fasten - detect and expose" .)