Big shady plant seabird's seen across ridge (4,4)
I believe the answer is:
tree fern
'big shady plant' is the definition.
'seabird's seen across ridge' is the wordplay.
'seabird' becomes 'tern' (I've seen this before).
'seen across' is an insertion indicator.
'ridge' becomes 'reef' (reef is a kind of ridge).
'tern' placed around 'reef' is 'TREE-FERN'.
(Other definitions for tree fern that I've seen before include "tropical plant" , "Tall plant" , "Palm-like plant" .)