Fungal growth, slight on endless sheep (6)
I believe the answer is:
mildew
'fungal growth' is the definition.
(I've seen this before)
'slight on endless sheep' is the wordplay.
'slight' becomes 'mild'.
'on' says to put letters next to each other.
'endless' means to remove the last letter.
'sheep' becomes 'ewe' (I've seen this before).
'ewe' with its final letter removed is 'ew'.
'mild'+'ew'='MILDEW'
(Other definitions for mildew that I've seen before include "Fungal coating on plant" , "Fungal growth on plants" , "Fungal disease in plants" , "White powdery fungus" , "rot" .)