Head gardener getting blue to shine (4)
I believe the answer is:
glow
'to shine' is the definition.
(I know that glow is a more specific form of the action beam)
'head gardener getting blue' is the wordplay.
'head' suggests taking the first letters.
'getting' is a charade indicator (letters next to each other).
'blue' becomes 'low' (I've seen this in another clue).
The initial letter of 'gardener' is 'g'.
'g'+'low'='GLOW'
(Other definitions for glow that I've seen before include "rosy colour" , "Shine steadily and brightly" , "Redness of the cheeks" , "Emit light and heat without flame" , "warmth of feeling" .)