Lying between two rivers, what was that industrial region? (4)
I believe the answer is:
ruhr
'industrial region?' is the definition.
(I've seen this before)
'lying between two rivers what was that' is the wordplay.
'two rivers' means two substitutions for 'river'.
'lying between' indicates putting letters inside.
'river' becomes 'r'.
'river' becomes 'r'.
'what was that' becomes 'uh' (sound used to ask someone to repeat themselves).
'r'+'r'='rr'
'rr' going around 'uh' is 'RUHR'.
(Other definitions for ruhr that I've seen before include "German industrial region" , "Ger. industrial region" , "German river flowing into the Rhine" , "Part of Germany" , "Rhine tributary, coal district" .)