Swell fellows standing in for us during holiday month? (7)
I believe the answer is:
Here is my best explanation:
'swell' is the definition.
'fellows standing in for us during holiday month?' is the subsidiary indication.
I cannot quite understand how this works, but
'fellows' could be 'men' (I've seen this before) and 'men' is present in the answer.
'month?' could be 'aug' (Aug is an example) and 'aug' is present in the answer.
A single letter 't' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for augment that I've seen before include "Make larger", "Make greater in size, number, etc.", "Add to, increase", "Increase, add to", "Enlarge or increase", "To increase", "Grow - intensify", "Expand" and "Supplement".)