Fish outside provided it's calm (7)
I believe the answer is:
mollify
'calm' is the definition.
(mollifying is a kind of calming)
'fish outside provided' is the wordplay.
'fish' becomes 'molly' (molly is a kind of fish**).
'outside' indicates putting letters inside.
'provided' becomes 'if'.
'molly' placed around 'if' is 'MOLLIFY'.
'it's' acts as a link.
(Other definitions for mollify that I've seen before include "Placate, appease" , "Soothe feelings of" , "Appease the anger of (someone)" .)