Dull, sort of like pool? (7)
I believe the answer is:
tarnish
'dull' is the definition.
(I've seen this before)
'sort of like pool?' is the wordplay.
I cannot really understand how this works, but
'like' could be 'ish' (similar in meaning) and 'ish' is present in the answer.
'pool?' could be 'tarn' (I've seen this in another clue) and 'tarn' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tarnish that I've seen before include "Stain, metal or reputation" , "Cause to lose lustre, silver or reputation" , "Stain like silver" , "Go dull" , "Stain or blemish, perhaps a reputation" .)