One unit in a space probe (7)
I believe the answer is:
pioneer
'space probe' is the definition.
(thesaurus)
'one unit in a' is the wordplay.
'one' becomes 'i' (Roman numeral).
'unit' becomes 'one' ('one' can be a synonym of 'unit'**).
'in' means one lot of letters goes inside another.
'a' becomes 'per'.
'i'+'one'='ione'
'ione' put within 'per' is 'PIONEER'.
(Other definitions for pioneer that I've seen before include "Lead the way" , "Innovator; first settler" , "Open up an area" , "make the first move!" , "One who blazes the trail for others" .)