Slippery muddy goal is one letting in United (10)
I believe the answer is:
oleaginous
'slippery' is the definition.
(thesaurus)
'muddy goal is one letting in united' is the wordplay.
I cannot really understand how this works, but
'one' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
'in' is within the answer.
'united' could be 'u' (as in Man U, English football team) and 'u' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for oleaginous that I've seen before include "oily" , "fawning" .)