Security measure in flat by Strand (7)
I believe the answer is:
padlock
'security measure' is the definition.
(I've seen this before)
'flat by strand' is the wordplay.
'flat' becomes 'pad' (a person's home).
'by' says to put letters next to each other.
'strand' becomes 'lock' (I've seen this before).
'pad'+'lock'='PADLOCK'
'in' is the link.
(Other definitions for padlock that I've seen before include "Security device with key" , "Security fastener" , "Securing device" , "security measure" , "Detachable security device" .)