Double up for example in pain (6)
I believe the answer is:
twinge
'pain' is the definition.
(I know that twinge is a type of pain)
'double up for example' is the wordplay.
'double' becomes 'twin' (twin can mean someone's double).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'for example' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'eg' back-to-front is 'ge'.
'twin'+'ge'='TWINGE'
'in' is the link.
(Other definitions for twinge that I've seen before include "Spasm" , "A small stab of pain" , "Slight pain (of conscience?)" , "little pain" , "Pinch" .)