Tightest locks on street securing back of house (7)
I believe the answer is:
meanest
'tightest' is the definition.
('mean' can be a synonym of 'tight')
'locks on street securing back of house' is the wordplay.
'locks' becomes 'mane' (both can mean hair).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'street' becomes 'st' (abbreviation).
'securing' means one lot of letters goes inside another.
'back of' suggests the final letters.
The last letter of 'house' is 'e'.
'mane'+'st'='manest'
'manest' placed around 'e' is 'MEANEST'.
(Other definitions for meanest that I've seen before include "Most unpleasant" , "Most cruel" , "Most miserable" , "Most stingy and least generous" , "ultimate in shabbiness?" .)