Mammoth damaged leg circling a river (5)
I believe the answer is:
large
'mammoth' is the definition.
(similar in meaning)
'damaged leg circling a river' is the wordplay.
'damaged' is an anagram indicator.
'circling' indicates putting letters inside (circling can mean going around or containing).
'river' becomes 'r' (abbreviation for river).
'leg' with letters rearranged gives 'lge'.
'a'+'r'='ar'
'lge' enclosing 'ar' is 'LARGE'.
(Other definitions for large that I've seen before include "Comprehensive" , "Lager (anag.)" , "Lake" , "Little's late partner" , "Wide-ranging" .)