Exemption from having to cross ebbing river (7)
I believe the answer is:
freedom
'exemption' is the definition.
(synonyms)
'from having to cross ebbing river' is the wordplay.
'having to cross' indicates putting letters inside.
'ebbing' is a reversal indicator (to ebb can mean to go backwards).
'river' becomes 'dee' (River Dee).
'dee' written backwards gives 'eed'.
'from' placed around 'eed' is 'FREEDOM'.
(Other definitions for freedom that I've seen before include "Power to act without restraint" , "Lack of restrictions" , "novel" , "Political militant" , "Release" .)