Thanks for the pen, it's very nice (5)
I believe the answer is:
tasty
'very nice' is the definition.
'tasty' can be an answer for 'nice' (thesaurus). I'm not sure about the 'very' bit.
'thanks for the pen' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'for' is a charade indicator (letters next to each other).
'the pen' becomes 'sty' (both can mean an animal enclosure).
'ta'+'sty'='TASTY'
'it's' is the link.
Can you help me to learn more?
(Other definitions for tasty that I've seen before include "Having good flavour" , "Savoury" , "Pleasantly flavoursome" , "Appetizing" , "Palatable" .)