Field for guinea pig? (7)
I believe the answer is:
subject
This clue is a double definition.
'field' is the first definition.
(synonyms)
'guinea pig?' is the second definition.
(I've seen this before)
'for' acts as a link.
(Other definitions for subject that I've seen before include "One owing allegiance to monarch" , "topic of conversation?" , "Topic - branch of study - citizen" , "UK citizen?" , "Theme - one under rule" .)