Fish outside provided it's calm (7)
I believe the answer is:
Here is my best explanation:
'calm' is the definition.
(mollifying is a kind of calming)
'fish outside provided' is the subsidiary indication.
'fish' becomes 'molly' (molly is a kind of fish**).
'outside' indicates putting letters inside.
'provided' becomes 'if'.
'molly' enclosing 'if' is 'mollify'.
'it's' is the link.
(Other definitions for mollify that I've seen before include "Soothe feelings of", "Placate, appease" and "Appease the anger of (someone)".)