A special forces unit, late coming in, has trouble accepting party is finally over! (2,4,2,1,4)
I believe the answer is:
as dead as a dodo
'is finally over' is the definition.
(as dead as a dodo means completely dead)
'a special forces unit late coming in has trouble' is the wordplay.
'special forces' becomes 'sas' (I've seen this in other clues).
'unit' becomes 'A' (A is an example).
'late' becomes 'dead' (similar in meaning).
'coming in' means one lot of letters goes inside another.
'has trouble' becomes 'dodo' (I can't explain this - if you can you should believe this answer much more).
'sas'+'a'='sasa'
'sasa' enclosing 'dead' is 'sdeadasa'.
'a'+'sdeadasa'+'dodo'='AS DEAD AS A DODO'
'accepting party' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
(Other definitions for as dead as a dodo that I've seen before include "Passed away (2,4,2,1,4)" , "No longer of the slightest use (2,4,2,1,4)" .)