Long-legged insect big bird observed on rush (5,3)
I believe the answer is:
crane fly
'long-legged insect' is the definition.
'crane fly' can be an answer for 'insect' (crane fly is a kind of insect). I'm unsure of the 'long-legged' bit.
'big bird observed on rush' is the wordplay.
'big bird' becomes 'crane' (crane is a kind of bird. I am not sure about the 'big' bit.).
'observed on' means one lot of letters go next to another (in a down clue, some letters go on top of others).
'rush' becomes 'fly' (both can mean to hurry).
'crane'+'fly'='CRANE-FLY'
Can you help me to learn more?
(Other definitions for crane fly that I've seen before include "One with long legs" , "Creature" , "Insect" , "A formal daddy-long-legs" .)