Understand about catching large fish (4)
I believe the answer is:
eels
'fish' is the definition.
(I've seen this before)
'understand about catching large' is the wordplay.
'understand' becomes 'see' (synonyms).
'about' shows that the letters should be reversed in order.
'catching' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'see' back-to-front is 'ees'.
'ees' enclosing 'l' is 'EELS'.
(Other definitions for eels that I've seen before include "Snakes wriggling in lees" , "Snakes found in lees" , "Else (anag.)" , "Feels small imprisoning water creatures" , "Lengthy fish" .)