Everyone is deceived about small item in fisherman’s catch (5,4)

'item in fisherman's catch' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'everyone is deceived about small' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'deceived' becomes 'had' (I've seen this in other clues).
'about' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'had' after 's' is 'shad'.
'all'+'is'+'shad'='ALLIS SHAD'

Can you help me to learn more?