Instructed, say, to be tense (4)
I believe the answer is:
taut
'be tense' is the definition.
(similar in meaning)
'instructed say' is the wordplay.
'instructed' becomes 'taught' (synonyms).
'say' shows a homophone (sound like).
'taught' is a homophone of 'TAUT'.
'to' acts as a link.
(Other definitions for taut that I've seen before include "Stretched, pulled tight" , "Held in tension" , "seaworthy" , "Tightly drawn" , "Fully stretched" .)