Stick to a present with inset diamonds (6)
I believe the answer is:
adhere
'stick to' is the definition.
('adhere' can be a synonym of 'stick to')
'a present with inset diamonds' is the wordplay.
'a present' becomes 'ad'.
'with' means one lot of letters go next to another.
'inset diamonds' becomes 'here' (I can't justify this - if you can you should give a lot more credence to this answer).
'ad'+'here'='ADHERE'
Can you help me to learn more?
(Other definitions for adhere that I've seen before include "secure" , "Hold fast, stand by" , "Bond" , "Stick fast (to something)" , "paste" .)