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 am not sure about the 'an attachment' bit.
'with happiness at heart' is the wordplay.
'at heart' says the answer is hidden in the clue (letters hidden in the heart of the clue).
'PIN' is hidden in the letters of 'with happiness'.
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" .)