Theatre in which audience begins to gather (4)
I believe the answer is:
reap
'gather' is the definition.
(I know that reap is a more specific form of the action gather)
'theatre in which audience begins' is the wordplay.
'theatre' becomes 'rep' (repertory theatre).
'in which' means one lot of letters goes inside another.
'begins' indicates taking the first letters.
The first letter of 'audience' is 'a'.
'rep' enclosing 'a' is 'REAP'.
'to' is the link.
(Other definitions for reap that I've seen before include "Save harvest, glean" , "''As you sow, so you . . . .'' (4)" , "Derive as a reward" , "Cut and gather (crop)" , "Harvest (a crop)" .)