Boy trapped back in concrete pillar (5)
I believe the answer is:
peter
'boy' is the definition.
'trapped back in concrete pillar' is the wordplay.
'back' shows that the letters should be reversed in order.
'trapped' indicates the answer is hidden within the clue.
'retep' is hidden within 'in concrete pillar'.
'retep' backwards is 'PETER'.
(Other definitions for peter that I've seen before include "Blue -, ship's flag" , "In whist, play" , "and 4 across Doctor Who actor" , "Saint's name" , "Sir - - Ustinov" .)