Deadly army in Cornish river (5)
I believe the answer is:
fatal
'deadly' is the definition.
('fatal' can be similar in meaning to 'deadly')
'army in cornish river' is the wordplay.
'army' becomes 'ta' (abbreviation for Territorial Army).
'in' means one lot of letters goes inside another.
'cornish river' becomes 'fal' (River Fal).
'ta' inserted inside 'fal' is 'FATAL'.
(Other definitions for fatal that I've seen before include "making one late" , "Deadly force" , "Pernicious" , "Causing death, say accident" , "Disastrous, maybe lethal" .)