Strip is in this manner hiding a bit of muck (7)
I believe the answer is:
isthmus
'strip' is the definition.
(I've seen this before)
'is in this manner hiding a bit of muck' is the wordplay.
'in this manner' becomes 'thus' (I've seen this before).
'hiding' indicates putting letters inside.
'a bit of' suggests taking the first letters.
The initial letter of 'muck' is 'm'.
'thus' placed around 'm' is 'thmus'.
'is'+'thmus'='ISTHMUS'
(Other definitions for isthmus that I've seen before include "Narrow land link" , "Narrow strip of land in sea" , "Narrow strip joining larger areas of land" , "Narrow strip linking two larger areas of land" , "Narrow land bridge" .)