Early poet’s lad catching salmon or sea trout (7)
I believe the answer is:
skelton
'early poet's' is the definition.
(15th-century poet John Skelton)
'lad catching salmon or sea trout' is the wordplay.
'lad' becomes 'son' (lad can informally mean a young boy).
'catching' is an insertion indicator.
'salmon or sea trout' becomes 'kelt' (kelt means a salmon or sea trout).
'son' placed around 'kelt' is 'SKELTON'.
(Other definitions for skelton that I've seen before include "Old poet" , "John --, English poet and tutor to Henry VIII" , "John -, English poet who tutored Henry VIII" .)