Soldier returns one strange American rock type (7)
I believe the answer is:
igneous
'rock type' is the definition.
(I've seen this before)
'soldier returns one strange american' is the wordplay.
'soldier' becomes 'GI' (term for US soldier).
'returns' shows that the letters should be reversed in order.
'strange' is an anagram indicator.
'american' becomes 'US' (United States).
'gi' reversed gives 'ig'.
'one' is an anagram of 'neo'.
'ig'+'neo'+'us'='IGNEOUS'
(Other definitions for igneous that I've seen before include "(Rock) of volcanic origin" , "(Of rock) solidified from a molten state" , "Type of rock from molten magma" , "Like fire - type of rock" , "Produced by the action of intense heat" .)