Cap has some slime on it (5)
I believe the answer is:
limit
'cap' is the definition.
(I know that cap is a more specific form of the action limit)
'some slime on it' is the wordplay.
'some' indicates the central letters.
'on' means one lot of letters go next to another.
The central letters of 'slime' are 'lim'.
'lim'+'it'='LIMIT'
'has' is the link.
(Other definitions for limit that I've seen before include "Ceiling" , "Constrain" , "Curtail" , "As far as something can go" , "Utmost" .)