Relax, sweetheart, when the sea is rough (4)
I believe the answer is:
ease
'relax' is the definition.
(eg to ease a restriction is to relax it)
'sweetheart when the sea is rough' is the wordplay.
'sweetheart when' becomes 'e' (I can't explain this - if you can you should give a lot more credence to this answer).
'is rough' is an anagram indicator (a rough or broken ordering of the letters).
'sea' anagrammed gives 'ase'.
'e'+'ase'='EASE'
Can you help me to learn more?
(Other definitions for ease that I've seen before include "Freedom from worry" , "Freedom from pain or anxiety" , "State of relaxation or rest" , "Reduce (pain)" , "Become less severe; facilitate" .)