Form an attachment with happiness at heart (3)
I believe the answer is:
pin
'form an attachment' is the definition.
'pin' can be an answer for 'form' (I have seen 'form of identification' mean 'pin' so perhaps 'form' could also mean 'pin'). I'm not certain of the 'an attachment' bit.
'happiness at heart' is the wordplay.
'at heart' says to take the centre.
The centre of 'happiness' is 'PIN'.
'with' acts as a link.
Can you help me to learn more?
(Other definitions for pin that I've seen before include "Fastener; brooch" , "form of identification" , "Peg or nail" , "Tack; fasten" , "Secure" .)