Supporting company agreement after some strain? That is indicator of future prospects (7,6)
I believe the answer is:
fortune cookie
'is indicator of future prospects' is the definition.
'supporting company agreement after some strain?' is the wordplay.
'supporting' becomes 'for'.
'company agreement' becomes 'cookie' (I can't justify this - if you can you should believe this answer much more).
'after' means one lot of letters go next to another.
'some strain?' becomes 'tune' (strain can mean the sound of a piece of music).
'cookie' put after 'tune' is 'tunecookie'.
'for'+'tunecookie'='FORTUNE COOKIE'
'that' is the link.
(Other definitions for fortune cookie that I've seen before include "Biscuit containing a message" , "Chinese restaurant speciality" , "Biscuit containing a prophecy" , "Pastry containing maxim" , "Small biscuit containing a prediction" .)