Slipping on hard fish (7)
I believe the answer is:
herring
'fish' is the definition.
(herring is a kind of fish)
'slipping on hard' is the wordplay.
'slipping' becomes 'erring' ('err' can be a synonym of 'slip').
'on' says to put letters next to each other.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'erring' after 'h' is 'HERRING'.
(Other definitions for herring that I've seen before include "Small silvery sea-fish" , "Fish that can be red and kippered!" , "Fatty fish" , "Saltwater fish" , "Common food fish" .)