Peril in the deep sewer, working inside it (3,7)
I believe the answer is:
sea monster
'peril in the deep sewer' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'working inside it' is the wordplay.
'working' becomes 'on' (e.g. a machine is working or on).
'inside' means one lot of letters goes inside another.
'it' becomes 'seamster' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' placed into 'seamster' is 'SEA-MONSTER'.
Can you help me to learn more?
(Other definitions for sea monster that I've seen before include "fabulous swimmer" , "dangerous creature in the main" , "A danger to mariners" .)