Repair piping blocked by bit of muck (6)
I believe the answer is:
remedy
'repair' is the definition.
(thesaurus)
'piping blocked by bit of muck' is the wordplay.
'piping' becomes 'reedy' (both can mean thin or high-pitched).
'blocked by' means one lot of letters goes inside another (block can mean to plug a gap in something).
'bit of' suggests taking the first letters.
The first letter of 'muck' is 'm'.
'reedy' enclosing 'm' is 'REMEDY'.
(Other definitions for remedy that I've seen before include "Something that cures, disease or problem" , "Medication" , "Put right, be a cure for" , "Nostrum" , "Treatment; redress" .)