Muster everybody in the outskirts of Rugby (5)
I believe the answer is:
rally
'muster' is the definition.
(I know that muster can be written as rally)
'everybody in the outskirts of rugby' is the wordplay.
'everybody' becomes 'all' (all people).
'in the' means one lot of letters goes inside another.
'outskirts of' suggests removing the centre (only letters on the outskirts).
'rugby' with its centre removed is 'ry'.
'all' inserted into 'ry' is 'RALLY'.
(Other definitions for rally that I've seen before include "Revive - assembly" , "Call to arms - series of strokes" , "Motor sport event" , "Recover; car race" , "Revive - motoring contest" .)