Force boy to be happy (4)
I believe the answer is:
glad
'happy' is the definition.
(I know that happy can be written as glad)
'force boy' is the wordplay.
'force' becomes 'G' (short for gravity).
'boy' becomes 'lad' (informal term for boy).
'g'+'lad'='GLAD'
'to be' is the link.
(Other definitions for glad that I've seen before include "These rags are dressy clothes - happy?" , "happiness" , "Little flowering plant" , "Happy, cheerful" , "part of Iris's family" .)