Picture one GI covered in hair (7)
I believe the answer is:
imagine
'picture' is the definition.
(picture can mean to visualise or imagine)
'one gi covered in hair' is the wordplay.
'one' becomes 'i' (Roman numeral).
'covered in' is an insertion indicator.
'hair' becomes 'mane' (mane is a kind of hair).
'gi' put within 'mane' is 'magine'.
'i'+'magine'='IMAGINE'
(Other definitions for imagine that I've seen before include "Form a mental image in the mind" , "Conjure up in mind,fancy" , "Believe to be so, suppose" , "Picture in mind, envisage" , "Fancy - dream" .)