Clamour about sealing off derelict pit is difficult to understand (7)
I believe the answer is:
cryptic
'difficult to understand' is the definition.
(as in a cryptic message)
'clamour about sealing off derelict pit' is the wordplay.
I cannot quite see how this works, but
'clamour' could be 'cry' (clamour can mean to cry or yell) and 'cry' is present in the answer.
'about' could be 'c' (abbreviation for 'circa') and 'c' is found within the answer.
an anagram of 'pit' is 'pti' which is present in the answer.
No letters remain.
This explanation may well be incorrect...
'is' acts as a link.
(Other definitions for cryptic that I've seen before include "Obscure or puzzling (as our page neighbour Crosaire)" , "Hidden" , "Obscure or puzzling (as our neighbour Crossheir)" , "Like our other puzzle" , "Esoteric" .)