Number like one thousand, primarily (5)
I believe the answer is:
digit
'number' is the definition.
(digit is a kind of whole number)
'like one thousand primarily' is the wordplay.
'like' becomes 'dig' (I've seen this in another clue).
'one' becomes 'i' (Roman numeral).
'primarily' suggests taking the first letters.
The initial letter of 'thousand' is 't'.
'dig'+'i'+'t'='DIGIT'
(Other definitions for digit that I've seen before include "Need a bit of a hand" , "finger, perhaps" , "Finger; number" , "that's on hand" , "one of several at hand" .)