Jittery Poles protected by supporter on course (5)
I believe the answer is:
tense
'jittery' is the definition.
(similar in meaning)
'poles protected by supporter on course' is the wordplay.
'poles' becomes 'ns' (north and south poles).
'protected by' means one lot of letters goes inside another.
'supporter on course' becomes 'tee' (a tee supports a ball on a golf course).
'ns' inserted within 'tee' is 'TENSE'.
(Other definitions for tense that I've seen before include "imperfect, perhaps?" , "Under emotional strain" , "Nerve-racking" , "under stress?" , "Nervous or tight" .)