Sanctimonious learner caught formidable fish (8)
I believe the answer is:
pilchard
'fish' is the definition.
(pilchard is a kind of fish)
'sanctimonious learner caught formidable' is the wordplay.
'sanctimonious' becomes 'pi'.
'learner' becomes 'l'.
'caught' becomes 'c' (cricket abbreviation).
'formidable' becomes 'hard' (similar in meaning).
'pi'+'l'+'c'+'hard'='PILCHARD'
(Other definitions for pilchard that I've seen before include "Item of seafood" , "'Small fatty fish, usually canned (8)'" , "Small fatty fish, usually tinned" , "type of fish" , "Small sea fish related to the herring" .)