One shouldn't pick this small taxi (4)
I believe the answer is:
scab
'one shouldn't pick this small taxi' is the definition.
The answer and definition can be both body parts as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'small taxi' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'taxi' becomes 'cab' (synonyms).
's'+'cab'='SCAB'
Can you help me to learn more?
(Other definitions for scab that I've seen before include "Crust forming over wound" , "Blackleg (colloq.)" , "Derogatory term for a strike-breaker" , "evidence of recent injury" , "One regarded with contempt" .)