Common character shown by soldiers in old company (7)
I believe the answer is:
orinoco
'common character' is the definition.
(I've seen this before)
'soldiers in old company' is the wordplay.
'soldiers' becomes 'OR' (Other Ranks).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'company' becomes 'co' (abbreviation).
'or'+'in'+'o'+'co'='ORINOCO'
'shown by' acts as a link.
(Other definitions for orinoco that I've seen before include "South-American river" , "O, I croon about the river" , "Colombia/Venezuela river" , "may be seen running through South America" , "Great river flowing through Venezuela to Atlantic" .)