Stretched when instructed, say (4)
I believe the answer is:
taut
'stretched' is the definition.
(I've seen this in another clue)
'instructed say' is the wordplay.
'instructed' becomes 'taught' ('teach' can be a synonym of 'instruct').
'say' shows a homophone (sound like).
'taught' is a homophone of 'TAUT'.
'when' is the link.
(Other definitions for taut that I've seen before include "Pulled or drawn tight" , "Without slack" , "On the slope" , "Held in tension" , "Tense, stretched tight" .)