Huge nervous reaction potting one brown (7)
I believe the answer is:
titanic
'huge' is the definition.
(I've seen this in another clue)
'nervous reaction potting one brown' is the wordplay.
I cannot really see how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is found in the answer.
'nervous reaction' could be 'tic' (I've seen this in another clue) and 'tic' is present in the leftover letters.
The remaining letters 'ti' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for titanic that I've seen before include "Twenty one [SHIP]" , "Of great force or power, like the ill-fated ship" , "Of great size, like a sunken ship" , "Extremely large" , "Hugely strong (but lost!)" .)