End of reinforced beam set back a few feet (4)
I believe the answer is:
yard
'a few feet' is the definition.
(I've seen this before)
'end of reinforced beam set back' is the wordplay.
'end of' says to take the final letters.
'beam' becomes 'ray' (synonyms).
'set back' says the letters should be written in reverse.
The final letter of 'reinforced' is 'd'.
'd'+'ray'='dray'
'dray' backwards is 'YARD'.
(Other definitions for yard that I've seen before include "Imperial unit of length" , "Measure - enclosed paved area" , "Dray (anag.)" , "Thirty six inches" , "Get the measure" .)