In that case the hand had got lost (4)
I believe the answer is:
then
'in that case' is the definition.
(synonyms)
'the hand had got lost' is the wordplay.
'had got' becomes 'had' ('have' can be a synonym of 'have got').
'lost' is a deletion indicator.
'hand' with 'had' taken away is 'n'.
'the'+'n'='THEN'
(Other definitions for then that I've seen before include "soon?" , "Consequently" , "Subsequently - previously" , "At that time; next" , "Once upon a time" .)