Great big fellows in semi-undress (7)
I believe the answer is:
immense
'great' is the definition.
(immense can mean great or huge)
'big fellows in semi-undress' is the wordplay.
I cannot quite see how this works, but
'fellows' could be 'men' (fellow can mean a man) and 'men' is located in the answer.
an anagram of 'semi' is 'imse' which is within the left over letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for immense that I've seen before include "Very big, exceedingly great" , "Huge, very big indeed" , "Very big, really huge" , "Huge, vast" , "Exceedingly large" .)