Calm producing uncertainty in fleet (6)
I believe the answer is:
pacify
'calm' is the definition.
(pacifying is a kind of calming)
'uncertainty in fleet' is the wordplay.
'uncertainty' becomes 'if' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'fleet' becomes 'pacy' (both can mean quick).
'if' put inside 'pacy' is 'PACIFY'.
'producing' acts as a link.
(Other definitions for pacify that I've seen before include "Soothe - appease" , "Bring peace to (a place)" , "Placate, calm" , "Make calm, appease" , "Quell the anger of" .)