Emphasise small lock of hair (6)
I believe the answer is:
stress
'emphasise' is the definition.
(I've seen this before)
'small lock of hair' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'lock of hair' becomes 'tress' (I've seen this before).
's'+'tress'='STRESS'
(Other definitions for stress that I've seen before include "'Strain, tension (6)'" , "what causes tension" , "Emphasise the pressure" , "Strain or accentuate" , "Pressure or strain" .)