Force boy to be happy (4)
I believe the answer is:
glad
'be happy' is the definition.
(synonyms)
'force boy' is the wordplay.
'force' becomes 'G' (short for gravity).
'boy' becomes 'lad' (informal term for boy).
'g'+'lad'='GLAD'
'to' 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" .)