Dear — a thousand for instrument that's not the largest (4,5)
I believe the answer is:
baby grand
'instrument that's not the largest' is the definition.
I can't judge whether this defines the answer.
'dear a thousand' is the wordplay.
'dear' becomes 'baby' (I can't explain this - if you can you should believe this answer much more).
'a thousand' becomes 'grand' ('grand' can be a synonym of 'thousand').
'baby'+'grand'='BABY GRAND'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for baby grand that I've seen before include "Small, non-upright piano" , "message of proud new parents?" , "Type of piano" , "Smaller piano" , "Infant piano?" .)