Only one in projecting window? Twaddle! (7)
I believe the answer is:
baloney
'twaddle' is the definition.
(synonyms)
'only one in projecting window?' is the wordplay.
'only' becomes 'lone' (synonyms).
'one in' means one lot of letters goes inside another.
'projecting window?' becomes 'bay' (bay window is a kind of window. I am not sure about the 'projecting' bit.).
'lone' inserted into 'bay' is 'BALONEY'.
Can you help me to learn more?
(Other definitions for baloney that I've seen before include "deceptive talk" , "Cobblers" , "foolish talk!" , "rhubarb" , "Nonsense; lean yob (anag)" .)