Blockhead beginning to lure in fish (4)
I believe the answer is:
clod
'blockhead' is the definition.
(thesaurus)
'beginning to lure in fish' is the wordplay.
'beginning to' indicates taking the first letters.
'in' indicates putting letters inside.
'fish' becomes 'cod' (cod is a kind of fish).
The initial letter of 'lure' is 'l'.
'l' placed into 'cod' is 'CLOD'.
(Other definitions for clod that I've seen before include "Fool - lump of earth" , "Stupid person - compact lump" , "Fool - bit of turf" , "Lump of mud" , "dunce" .)