Inflexibly cunning, snaring seabird (7)
I believe the answer is:
sternly
'inflexibly' is the definition.
Both the definition and answer are adverbs. Perhaps you can see a link between them that I can't see?
'cunning snaring seabird' is the wordplay.
'cunning' becomes 'sly' (synonyms).
'snaring' indicates putting letters inside.
'seabird' becomes 'tern' (tern is a kind of seabird).
'sly' enclosing 'tern' is 'STERNLY'.
Can you help me to learn more?
(Other definitions for sternly that I've seen before include "Cruelly" , "In serious way" .)