Diluted for seven days, say (4)
I believe the answer is:
weak
'diluted' is the definition.
(similar in meaning)
'seven days say' is the wordplay.
'seven days' becomes 'week' (I've seen this before).
'say' indicates a 'sounds like' (homophone) clue.
'week' is a homophone of 'WEAK'.
'for' acts as a link.
(Other definitions for weak that I've seen before include "Lacking strength" , "Undefended" , "Feeble or frail" , "Faint; frail" , "lacking intensity?" .)