David's wife is the lady plugging city travel firm (9)
I believe the answer is:
bathsheba
'david's wife' is the definition.
(wife of King David in the Bible)
'is the lady plugging city travel firm' is the wordplay.
'is the lady' becomes 'she' ('she' refers to a woman).
'plugging' means one lot of letters goes inside another (inserted letters plug up a gap in the word).
'city' becomes 'bath' (English city).
'travel firm' becomes 'ba' (I am not sure about this - if you are sure you should believe this answer much more).
'bath'+'ba'='bathba'
'she' placed inside 'bathba' is 'BATHSHEBA'.
Can you help me to learn more?
(Other definitions for bathsheba that I've seen before include "who married David?" , "biblical character" , "female partner in affair" , "OT character" , "woman once exploited" .)