Flash beam by tree near banks of Tynemouth inlet (5,5)
I believe the answer is:
moray firth
'inlet' is the definition.
(I know that Moray Firth is a loch)
'flash beam by tree near banks of tynemouth' is the wordplay.
'flash' becomes 'mo' (both can mean a very short period of time).
'beam' becomes 'ray' (synonyms).
'by' means one lot of letters go next to another.
'tree near' becomes 'fir' (fir tree is a kind of tree. I am not sure about the 'near' bit.).
'banks of' says to hollow out the word (remove centre letters) (the banks are the sides of the word).
'tynemouth' with its middle taken out is 'th'.
'mo'+'ray'+'fir'+'th'='MORAY FIRTH'
(Other definitions for moray firth that I've seen before include "Scottish water" , "Inverness lies at its head" .)