Source of beneficial reflection — certainly cheers up in church (7)
I believe the answer is:
catseye
'source of beneficial reflection' is the definition.
(beneficial reflection when driving)
'certainly cheers up in church' is the wordplay.
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
'cheers' becomes 'ta' (both can informally mean thank you).
'up' is a reversal indicator (in down clue: letters go upwards).
'in' indicates putting letters inside.
'church' becomes 'CE' (Church of England).
'yes'+'ta'='yesta'
'yesta' in reverse letter order is 'atsey'.
'atsey' inserted within 'ce' is 'CATSEYE'.
(Other definitions for catseye that I've seen before include "Road safety device" , "Reflector in the road" , "Middle-of-the-road reflector" .)