Slender and extremely greasy whatchamacallit (6)
I believe the answer is:
thingy
'whatchamacallit' is the definition.
(I've seen this in another clue)
'slender and extremely greasy' is the wordplay.
'slender' becomes 'thin' (synonyms).
'and' means one lot of letters go next to another.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'greasy' with its centre taken out is 'gy'.
'thin'+'gy'='THINGY'
(Other definitions for thingy that I've seen before include "What's-its-name - nighty (anag)" , "I forget his name" , "whose name can't be remembered" .)