Stretch of water requiring popular permit (5)
I believe the answer is:
inlet
'stretch of water' is the definition.
(a small area of water)
'popular permit' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'permit' becomes 'let' (synonyms).
'in'+'let'='INLET'
'requiring' is the link.
(Other definitions for inlet that I've seen before include "Coastal feature" , "Opening - passage" , "Small bay or creek" , "Means of entry" , "Arm of lake, river" .)