You and I must be seen in fancy theatres, darling (10)
I believe the answer is:
sweetheart
'darling' is the definition.
(I've seen this in another clue)
'you and i must be seen in fancy theatres' is the wordplay.
'you and i' becomes 'we' (we means you and I).
'must be seen in' is an insertion indicator.
'fancy' is an anagram indicator.
'theatres' with letters rearranged gives 'setheart'.
'we' placed into 'setheart' is 'SWEETHEART'.
(Other definitions for sweetheart that I've seen before include "Lover" , "Beloved" , "Dear" , "Steady!" , "Darlin'"" .)