Missile launcher putting to sea leaves capital (5)
I believe the answer is:
sling
'missile launcher' is the definition.
(eg a slingshot)
'putting to sea leaves capital' is the wordplay.
'putting to sea' becomes 'sailing' (sailing out to sea).
'leaves' is a deletion indicator.
'capital' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'sailing' with 'ai' taken away is 'SLING'.
(Other definitions for sling that I've seen before include "Sweetened drink with spirit base" , "attached to a rifle, perhaps" , "Arm bandage; hurl" , "harness for broken arm" , "Catapult - shoe - support" .)