Colour seen in half of pigs, in the main (5)
I believe the answer is:
sepia
'colour' is the definition.
(I know that sepia is a colour)
'half of pigs in the main' is the wordplay.
'half of' suggests halving the letters.
'in the' is an insertion indicator.
'main' becomes 'sea' (the main is an old name for the sea).
'pigs' halved is 'pi'.
'pi' placed inside 'sea' is 'SEPIA'.
'seen in' acts as a link.
(Other definitions for sepia that I've seen before include "Brown colour of old photo" , "cuttle" , "Denizen of deep" , "Old drawing" , "Former photographic pigment" .)