Lecture man on sex — husband (5)
I believe the answer is:
reith
'lecture man' is the definition.
(the Reith Lectures are a BBC lecture series)
'on sex husband' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'sex' becomes 'it' (a common euphemism).
'husband' becomes 'h'.
're'+'it'+'h'='REITH'
(Other definitions for reith that I've seen before include "BBC radio pioneer" , "Head of BBC" , "First BBC Director General" , "Lord --, BBC founder" , "head of broadcasting" .)