Actor with an interest to become established in short series (11)
I believe the answer is:
stakeholder
'actor with an interest' is the definition.
(someone with financial interest in something)
'become established in short series' is the wordplay.
'become established' becomes 'take hold' (taking hold is a kind of becoming established).
'in' indicates putting letters inside.
'short series' becomes 'ser' (abbreviation for series; 'short' suggests a short form).
'takehold' placed inside 'ser' is 'STAKEHOLDER'.
'to' is the link.
(Other definitions for stakeholder that I've seen before include "Someone with an interest" , "One with an interest (in something)" , "Person or organisation with an interest in something" , "kind of pension?" , "One who retains an interest" .)