Way of speaking recalled in time in letter (7)
I believe the answer is:
epsilon
'letter' is the definition.
(I know that epsilon is a type of letter)
'way of speaking recalled in time' is the wordplay.
'way of speaking' becomes 'lisp' (speech impediment).
'recalled' is a reversal indicator (to recall can mean to bring something back).
'in' is an insertion indicator.
'time' becomes 'eon' (eon is a kind of time).
'lisp' in reverse letter order is 'psil'.
'psil' put within 'eon' is 'EPSILON'.
'in' acts as a link.
(Other definitions for epsilon that I've seen before include "Foreign character" , "One's lip (anag)" , "character in Greece" , "Greek character" , "small number mispronounced, almost completely the wrong way round" .)