Oily liquid worryingly rose inside bird's shelter (8)
I believe the answer is:
creosote
'oily liquid' is the definition.
(liquid made from oil)
'worryingly rose inside bird's shelter' is the wordplay.
'worryingly' indicates an anagram (I've seen this in other clues).
'inside' is an insertion indicator.
'bird's shelter' becomes 'cote'.
'rose' is an anagram of 'reos'.
'reos' put within 'cote' is 'CREOSOTE'.
(Other definitions for creosote that I've seen before include "Fencing gear, perhaps" , "Wood treatment" , "Preserve deal, say" , "Preservative liquid obtained from coal tar" , "Wood preservative oil" .)