I’d turned round in narrow opening (4-2)
I believe the answer is:
lead-in
'opening' is the definition.
(introductory section)
'id turned round in narrow' is the subsidiary indication.
'turned round' shows that the letters should be reversed in order.
'in' means one lot of letters goes inside another.
'narrow' becomes 'lean'.
'id' in reverse letter order is 'di'.
'di' going within 'lean' is 'leadin'.
(Other definitions for lead-in that I've seen before include "Introduction to subject" , "Introductory passage" , "Introduction, preamble" .)