Err again, by gum! (5)
I believe the answer is:
resin
'gum' is the definition.
(I've seen this in another clue)
'err again by' is the wordplay.
'err' becomes 'sin' (I've seen this before).
'again' becomes 're' (prefix meaning 'again').
'by' says to put letters next to each other.
'sin' after 're' is 'RESIN'.
(Other definitions for resin that I've seen before include "varnish, perhaps" , "Tree gum" , "Sticky stuff from pine trees" , "Sticky material" , "Exudation from fir trees" .)