Let eels wriggle out of one! (5)
I believe the answer is:
lease
'let' is the definition.
(as in to lease or let a building)
'eels wriggle out of one' is the wordplay.
'wriggle' indicates an anagram (I've seen 'wriggling' mean this (letters wriggle or move about)).
'out of' indicates putting letters inside (some letters on the outside of others).
'one' becomes 'a' (a thing is one thing).
'eels' anagrammed gives 'lese'.
'lese' enclosing 'a' is 'LEASE'.
(Other definitions for lease that I've seen before include "Summer's has too short a date (Shak.)" , "Letting contract" , "From legal document" , "Ghost" , "Tenancy agreement" .)