Small lock of hair causing anxiety (6)
I believe the answer is:
stress
'anxiety' is the definition.
(anxiety is stress or tension)
'small lock of hair' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'lock of hair' becomes 'tress' (I've seen this in another clue).
's'+'tress'='STRESS'
'causing' is the link.
(Other definitions for stress that I've seen before include "'Strain, tension (6)'" , "Feeling of pressure" , "Ictus" , "Dwell on" , "Emphasize" .)