Attempt to block chimney erected for tycoon (5)
I believe the answer is:
mogul
'for tycoon' is the definition.
(both can mean a rich person)
'attempt to block chimney erected' is the wordplay.
'attempt' becomes 'go' (go is a kind of attempt).
'to block' means one lot of letters goes inside another (upper walls off glass inside formal).
'chimney' becomes 'lum' (I've seen this in other clues).
'erected' is a reversal indicator (in a down clue, letters go up).
'go' going inside 'lum' is 'lugom'.
'lugom' reversed gives 'MOGUL'.
(Other definitions for mogul that I've seen before include "Powerful businessman" , "Important person; snow mound on a ski slope" , "big shot" , "bigwig" , "Powerful man" .)