Strain shown by, say, future head (9)
I believe the answer is:
tenseness
'strain' is the definition.
(tenseness is a kind of strain)
'shown by say future head' is the wordplay.
'shown by' means one lot of letters go next to another.
'say future' becomes 'tense' (I can't explain this - if you can you should believe this answer much more).
'head' becomes 'ness' (ness is a headland).
'tense'+'ness' is 'TENSENESS'.
Can you help me to learn more?
(Other definitions for tenseness that I've seen before include "Cosmetic lotion" , "Condition of being taut" .)