Concealed a number in the latter half (6)
I believe the answer is:
latent
'concealed' is the definition.
(I've seen this before)
'a number in the latter half' is the wordplay.
'a number' becomes 'ten' (ten is a number).
'in' means one lot of letters goes inside another.
'half' suggests halving the letters.
'latter' with half the letters taken is 'lat'.
'ten' put within 'lat' is 'LATENT'.
(Other definitions for latent that I've seen before include "Inactive - potential" , "Existing but not yet active or visible" , "Hidden (potential)" , "'Hidden, undeveloped (6)'" , "Hidden (talent)" .)