Large plant seabird observed crossing sandbank (4,4)
I believe the answer is:
'large plant' is the definition.
(type of large plant)
'seabird observed crossing sandbank' is the wordplay.
'seabird' becomes 'tern' (tern is a kind of seabird).
'observed crossing' means one lot of letters goes inside another (some letters go around or cross others).
'sandbank' becomes 'reef' (I've seen this before).
'tern' going around 'reef' is 'TREE-FERN'.
(Other definitions for tree fern that I've seen before include "Tall plant" , "tropical plant" , "Palm-like plant" .)