Set off around River Severn's first lock (5)
I believe the answer is:
tress
'lock' is the definition.
(both can mean a piece of hair)
'set off around river severn's first' is the wordplay.
'off' is an anagram indicator.
'around' is an insertion indicator.
'river' becomes 'r' (abbreviation).
'severn's first' becomes 's' (1st letter of 'severn').
'set' with letters rearranged gives 'tes'.
'tes' placed around 'r' is 'tres'.
'tres'+'s'='TRESS'
(Other definitions for tress that I've seen before include "Rests (anag.)" , "Braid of hair" , "is a bit of a shock" , "Long piece of hair" , "Bit of hair" .)