Yell, perhaps, or be quiet, sweetest heart: if air is air and sea is sea … (8,6)

'yell perhaps' is the definition.
(Yell is one of the Shetland Islands)

'be quiet sweetest heart if air is air and sea is sea' is the wordplay.
I cannot quite understand how this works, but
'quiet' could be 'sh' and 'sh' is found within the answer.
'is' is found in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is present in the answer.
This may be the basis of clue (or it may be nonsense).

'or' acts as a link.