Small lock causing tension (6)
I believe the answer is:
stress
'tension' is the definition.
(synonyms)
'small lock' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'lock' becomes 'tress' (both can mean a piece of hair).
's'+'tress'='STRESS'
'causing' acts as a link.
(Other definitions for stress that I've seen before include "result of being overworked" , "Emphasis on anxiety?" , "suffering" , "Emphasis: result often of being under pressure" , "Tension - accent" .)