Even reversing right in - the cheek! (5)
I believe the answer is:
nerve
'the cheek' is the definition.
(synonyms)
'even reversing right in' is the wordplay.
'reversing' is a reversal indicator.
'right' becomes 'r' (common abbreviation).
'in' indicates putting letters inside.
'even' reversed gives 'neve'.
'neve' placed around 'r' is 'NERVE'.
(Other definitions for nerve that I've seen before include "Impulse transmitter - bottle" , "Fibre carrying impulses from brain to body" , "Courage, coolness in danger" , "Intrepidity" , "Courage, like bodily fibre" .)