Feel sure they are difficult to catch inside (4)
I believe the answer is:
eels
'catch inside' is the definition.
'eels' can be an answer for 'catch' (eel is a type of fish). I am not certain of the 'inside' bit.
'feel sure they are difficult' is the wordplay.
'EELS' is hidden in amongst the letters.
However, I am unsure how this is indicated.
'to' is the link.
Can you help me to learn more?
(Other definitions for eels that I've seen before include "Fish (that may be jellied)" , "Feels small imprisoning water creatures" , "(Slippery) fish" , "Snakes found in lees" , "Snakes wriggling in lees" .)