Pressure from small lock (6)
I believe the answer is:
stress
'pressure' is the definition.
(I've seen this in another clue)
'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'
'from' is the link.
(Other definitions for stress that I've seen before include "Tension, pressure" , "'Strain, tension (6)'" , "Emphasis on anxiety?" , "Feeling of pressure" , "Physical or mental over-exertion" .)