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' says to hollow out the word (remove centre 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' indicates putting letters inside (inserted letters are sandwiched in).
'cheerful' with its middle removed is 'cl'.
'f' going inside '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" .)