Small lock causes anxiety (6)
I believe the answer is:
stress
'anxiety' is the definition.
(I've seen this before)
'small lock' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'lock' becomes 'tress' (I've seen this before).
's'+'tress'='STRESS'
'causes' is the link.
(Other definitions for stress that I've seen before include "suffering" , "Physical or mental over-exertion" , "Lay emphasis on - it's a strain" , "Emotional tension" , "State mental or emotional strain" .)