Gouda in box occasionally includes fertiliser (5)
I believe the answer is:
guano
'fertiliser' is the definition.
(I've seen this before)
'gouda in box occasionally' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'gouda'+'in'+'box'='goudainbox'
The alternating letters of 'goudainbox' are 'GUANO'.
'includes' acts as a link.
(Other definitions for guano that I've seen before include "Bird droppings used as fertiliser" , "Fertiliser (either from bird droppings or artificial)" , "Natural fertiliser" , "type of fertiliser" , "mess about by the sea" .)