Require sweetheart to hold left hand (6)
I believe the answer is:
needle
'hand' is the definition.
(thesaurus)
'require sweetheart to hold left' is the wordplay.
'require' becomes 'need' (synonyms).
'sweetheart' becomes 'e' (the middle letter or heart of 'sweet').
'to hold' indicates putting letters inside.
'left' becomes 'l' (common abbreviation).
'need'+'e'='neede'
'neede' placed around 'l' is 'NEEDLE'.
(Other definitions for needle that I've seen before include "sledge?" , "Long thin leaf of pine tree" , "Continually annoy" , "Leaf of a 15 across" , "Sewing implement used to annoy" .)