Looking up numbers seems crazy but it might bear fruit (6)
I believe the answer is:
damson
'it might bear fruit' is the definition.
(I know that fruit tree often perform the action bear a fruit)
'looking up numbers seems crazy' is the wordplay.
'looking up' shows that the letters should be reversed in order (in a down clue, letters go up).
'numbers' becomes 'nos' ('no' is an abbreviation for number).
'seems crazy' becomes 'mad' (I can't explain this - if you can you should believe this answer much more).
'nos'+'mad'='nosmad'
'nosmad' written backwards gives 'DAMSON'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for damson that I've seen before include "Small fruit for nomads" , "Small stone-fruit for nomads" , "fruit tree" , "Acid purple plum" , "might one be in a jam?" .)