Unknown number in a wood, together (5)
I believe the answer is:
along
'together' is the definition.
(I've seen this before)
'unknown number in a wood' is the wordplay.
'unknown number' becomes 'n'.
'in' is an insertion indicator.
'wood' becomes 'log' (a log is a piece of wood).
'a'+'log'='alog'
'n' put inside 'alog' is 'ALONG'.
(Other definitions for along that I've seen before include "From one end to the other" , "Moving in a constant direction on (something)" , "(Going) in company (with others)" , "By the side of" , "Sea monster, opposite Charybdis" .)