On reflection, it's not possible to keep current Head (5)
I believe the answer is:
onion
'head' is the definition.
(informal term for someone's head or brain)
'on reflection it's not possible to keep current' is the wordplay.
'on reflection' is a reversal indicator.
'it's not' becomes 'no' (negative).
'possible' becomes 'on' ('not on' can mean unacceptable or not possible).
'to keep' indicates putting letters inside.
'current' becomes 'I' (symbol for electric current).
'no' in reverse letter order is 'on'.
'on'+'on'='onon'
'onon' going around 'i' is 'ONION'.
(Other definitions for onion that I've seen before include "Head (slang); vegetable" , "Tear-jerking bulb" , "one understood by expert" , "reason for tears" , "Tear-inducing edible bulb" .)