As a rule, it's seen in foreigners (5)
I believe the answer is:
reign
'as a rule' is the definition.
(I've seen this before)
'seen in foreigners' is the wordplay.
'seen in' indicates the answer is hidden within the clue.
'REIGN' is hidden within 'foreigners'.
'it's' is the link.
(Other definitions for reign that I've seen before include "Sovereignty" , "Sit on a throne" , "Rule as king or queen" , "Period of the rule of a sovereign" , "Hold sway, rule" .)