Look at what's in popular detergent (6)
I believe the answer is:
behold
'look at' is the definition.
(I've seen this before)
'what's in popular detergent' is the wordplay.
'what' becomes 'eh' (sound used to ask 'what?' or 'pardon?').
'in' is an insertion indicator.
'popular detergent' becomes 'bold' (I can't explain this - if you can you should believe this answer much more).
'eh' going within 'bold' is 'BEHOLD'.
Can you help me to learn more?
(Other definitions for behold that I've seen before include "Look at with attention" , "See! Look!" , "See, with attention" , "Archaic injunction to look" , "Lo - look!" .)