Bluff, extremely cheerful fellow provided sandwiches (5)
I believe the answer is:
cliff
'bluff' is the definition.
(bluff can mean a cliff)
'extremely cheerful fellow provided sandwiches' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'fellow' becomes 'F' (abbreviation - of a society etc.).
'provided' becomes 'if' (eg 'provided it's sunny' means 'if it's sunny').
'sandwiches' means one lot of letters goes inside another (inserted letters are sandwiched in).
'cheerful' with its centre taken out is 'cl'.
'f' going into 'if' is 'iff'.
'cl'+'iff'='CLIFF'
(Other definitions for cliff that I've seen before include "stony face?" , "Steep high rockface" , "Sir - - Richard" , "A sheer rock face, in mountain or by sea" , "A sheer rock-face" .)