..... otherwise bowler back about Pentateuch (5)
I believe the answer is:
torah
'pentateuch' is the definition.
(I've seen this in another clue)
'otherwise bowler back about' is the wordplay.
'otherwise' becomes 'or'.
'bowler' becomes 'hat' (I've seen this in another clue).
'back' says the letters should be written in reverse.
'about' is an insertion indicator.
'hat' in reverse letter order is 'tah'.
'or' placed within 'tah' is 'TORAH'.
(Other definitions for torah that I've seen before include "Pentateuch" , "The will of God as revealed in Mosaic law" , "The whole body of Jewish religious literature" , "Jewish law" , "Law as revealed to Moses (Judaism)" .)