Pressure on soldier to return swine (3)
I believe the answer is:
pig
'swine' is the definition.
(pig is a kind of swine)
'pressure on soldier to return' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'soldier' becomes 'gi' (term for US soldier).
'to return' says the letters should be written in reverse.
'gi' back-to-front is 'ig'.
'p'+'ig'='PIG'
(Other definitions for pig that I've seen before include "Eg, sow" , "Greedy person (colloq.)" , "& 16 Animal pen" , "Domestic swine" , "Eat greedily" .)