Letter records rampaging lion (7)
I believe the answer is:
epsilon
'letter' is the definition.
(epsilon is a kind of letter)
'records rampaging lion' is the wordplay.
'records' becomes 'eps' (EP is a type of record).
'rampaging' indicates an anagram (I've seen this in other clues).
'lion' is an anagram of 'ilon'.
'eps'+'ilon'='EPSILON'
(Other definitions for epsilon that I've seen before include "foreign letter" , "Fifth letter of the Greek alphabet" , "5th letter of the Greek alphabet" , "Foreign character" , "Greek character" .)