Anchor showing unusual stamina, say -- finally (8)
I believe the answer is:
Here is my best explanation:
'anchor' is the definition.
('mainstay' can be a synonym of 'anchor')
'unusual stamina say finally' is the subsidiary indication.
'unusual' indicates an anagram.
'finally' suggests the final letters.
The final letter of 'say' is 'y'.
'stamina' with letters rearranged gives 'mainsta'.
'showing' is the link.
(Other definitions for mainstay that I've seen before include "piece", "Linchpin, principal support", "Chief source of support", "Pillar of strength", "Strong cable that secures a mast, chief support generally", "Strongest support", "pillar", "Piece of rigging", "Rope to the mast is the chief support" and "Principal support".)