Grain seen reflected in moonrock (4)
I believe the answer is:
corn
'grain' is the definition.
(I know that corn is a type of kernel)
'reflected in moonrock' is the wordplay.
'reflected' is a reversal indicator.
'in' says the answer is hidden in the clue.
'nroc' can be found hidden inside 'moonrock'.
'nroc' in reverse letter order is 'CORN'.
'seen' is the link.
(Other definitions for corn that I've seen before include "Painful foot problem" , "Crop - growth" , "Cereal; grain" , "Maize" , "Painful affliction" .)