Long number that's secret and ending in three? (4)
I believe the answer is:
pine
'long' is the definition.
(to pine for someone is to long for them)
'number that's secret and ending in three?' is the wordplay.
'number that's secret' becomes 'PIN' (PIN, personal identification number).
'and' says to put letters next to each other.
'ending in' indicates one should take the final letters.
The last letter of 'three' is 'e'.
'pin'+'e'='PINE'
(Other definitions for pine that I've seen before include "A tree to grieve for" , "Fir tree; languish" , "Coniferous tree" , "Evergreen conifer" , "Tree, waste away" .)