Titled person needing oxygen in Greater London borough (7)
I believe the answer is:
baronet
'titled person' is the definition.
(someone with the title of baronet)
'oxygen in greater london borough' is the wordplay.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'in' means one lot of letters goes inside another.
'greater london borough' becomes 'barnet' (I've seen this in another clue).
'o' placed within 'barnet' is 'BARONET'.
'needing' acts as a link.
(Other definitions for baronet that I've seen before include "Bent oar (anag.)" , "Nobleman" , "Tone bar (anag.)" , "Minor title" , "Member of the British order of honour" .)