Unstructured mass survey needs sending back to party (6)
I believe the answer is:
dollop
'unstructured mass' is the definition.
(a dollop is a mass of a substance)
'survey needs sending back to party' is the wordplay.
'survey' becomes 'poll' (polling is a kind of surveying).
'needs sending back' is a reversal indicator.
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'party' becomes 'do' (do is a kind of party).
'poll' backwards is 'llop'.
'llop' put after 'do' is 'DOLLOP'.
(Other definitions for dollop that I've seen before include "Inelegant helping (of)" , "serving food, perhaps" , "Small measure, of mash say" , "Semisolid lump" , "Large shapeless lump" .)