Grains needing sun also (4)
I believe the answer is:
sand
'grains' is the definition.
(sand is made up of grains)
'sun also' is the wordplay.
'sun' becomes 'S' (abbreviation).
'also' becomes 'and' (both can mean in addition).
's'+'and'='SAND'
'needing' is the link.
(Other definitions for sand that I've seen before include "Content of hourglass" , "Grains of quartz" , "George -, Fr. author" , "Desert grit" , "Seaside castle-building material" .)