In period of darkness, blew a boat up (2,1,3,3)
I believe the answer is:
at a low ebb
'in period of darkness' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'blew a boat up' is the wordplay.
'up' indicates anagramming the letters (letters get chucked up).
'blew'+'a'+'boat'='blewaboat'
'blewaboat' anagrammed gives 'AT A LOW EBB'.
Can you help me to learn more?
(Other definitions for at a low ebb that I've seen before include "'fed up'" , "on the way out" , "In a poor state" , "temporarily down" , "when the beach is empty" .)