Hell -- could be clear the girl's promise crosses line (11)

'hell' is the definition.
(I know that hell can be written as netherworld)

'clear the girl's promise crosses line' is the wordplay.
'clear the girl's' becomes 'nether' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'promise' becomes 'word' (as in 'I give you my word').
'crosses' is an insertion indicator.
'line' becomes 'l' (used when specifying particular lines from a poem).
'word' going around 'l' is 'world'.
'nether'+'world'='NETHERWORLD'

'could be' acts as a link.

Can you help me to learn more?