Gauged tree flourishing in sea (7)
I believe the answer is:
metered
'gauged' is the definition.
(I've seen this before)
'tree flourishing in sea' is the wordplay.
'flourishing' indicates anagramming the letters (letters grow or flourish differently).
'in' means one lot of letters goes inside another.
'sea' becomes 'med' (abbreviation of Mediterranean).
'tree' with letters rearranged gives 'tere'.
'tere' going within 'med' is 'METERED'.
(Other definitions for metered that I've seen before include "Gauged" , "Measured" , "As water may be" .)