Puzzles force John Robins to lose heart (6)
I believe the answer is:
floors
'puzzles' is the definition.
'force john robins to lose heart' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'john' becomes 'loo' (both can mean a toilet).
'to lose heart' says to hollow out the word (remove centre letters) (I've seen 'losing heart' mean this).
'robins' with its centre taken out is 'rs'.
'f'+'loo'+'rs'='FLOORS'
(Other definitions for floors that I've seen before include "Inside horizontal surfaces" , "Storeys" , "Knocks person down in a fight or confounds" , "Stumps (with question)" , "Tiers in a building" .)