Half suck garish sweet substance (5)
I believe the answer is:
sugar
'sweet substance' is the definition.
(I've seen this before)
'half suck garish' is the wordplay.
'half' indicates taking half.
'garish' becomes 'gar' (I can't explain this - if you can you should believe this answer much more).
'suck' with half the letters taken is 'su'.
'su'+'gar'='SUGAR'
(Other definitions for sugar that I've seen before include "Plantation product" , "flattery" , "Mild oath (granulated?)" , "Sweeten" , "Argus (anag.)" .)