Stick a number in this space (6)
I believe the answer is:
adhere
'stick' is the definition.
(I know that stick can be written as adhere)
'a number in this space' is the wordplay.
'number' becomes 'D' (D is an example).
'in this space' becomes 'here' (I've seen this in another clue).
'a'+'d'+'here'='ADHERE'
(Other definitions for adhere that I've seen before include "Stick L" , "'Stick to, a play say (6)'" , "Stick or cleave to" , "Stick firmly" , "Keep" .)