Old boy's story about opening of cell block (8)
I believe the answer is:
obstacle
'block' is the definition.
(I've seen this before)
'old boy's story about opening of cell' is the wordplay.
'old boy's' becomes 'obs' (abbreviation).
'story' becomes 'tale' (synonyms).
'about' indicates putting letters inside.
'opening of' indicates taking the first letters.
The first letter of 'cell' is 'c'.
'tale' placed around 'c' is 'tacle'.
'obs'+'tacle'='OBSTACLE'
(Other definitions for obstacle that I've seen before include "Progress blocker" , "stumbling block" , "Block to progress" , "It hinders or prevents advance" , "Blockage, hindrance" .)