Plan to perform in an instant? Not quite (6)
I believe the answer is:
tactic
'plan' is the definition.
(I've seen this before)
'perform in an instant? not quite' is the wordplay.
'perform' becomes 'act' (acting is a kind of performing).
'in' means one lot of letters goes inside another.
'an instant?' becomes 'tick' (I've seen this before).
'not quite' means to remove the last letter.
'tick' with its last letter taken off is 'tic'.
'act' placed into 'tic' is 'TACTIC'.
'to' acts as a link.
(Other definitions for tactic that I've seen before include "An action planned to achieve a specific end" , "A TT C-i-C (anag.)" , "Military means to an end" , "Stratagem, procedure" , "Manoeuvre designed to outwit enemy" .)