Bird is spotted when inside hide (6)
I believe the answer is:
siskin
'bird' is the definition.
(siskin is a kind of bird)
'spotted when inside hide' is the wordplay.
'spotted when' becomes 'is' (I can't justify this - if you can you should believe this answer much more).
'inside' is an insertion indicator.
'hide' becomes 'skin' (synonyms).
'is' going inside 'skin' is 'SISKIN'.
'is' is the link.
Can you help me to learn more?
(Other definitions for siskin that I've seen before include "Small yellow and black finch" , "Winger" , "Yellow-and-black finch" , "Yellowish-green finch" , "Type of finch" .)