Swell fellows standing in for us during holiday month? (7)
I believe the answer is:
augment
'swell' is the definition.
(thesaurus)
'fellows standing in for us during holiday month?' is the wordplay.
I cannot quite see how this works, but
'fellows' could be 'men' (I've seen this before) and 'men' is found in the answer.
'month?' could be 'aug' (Aug is an example) and 'aug' is found in the answer.
A single letter 't' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for augment that I've seen before include "Add to, increase" , "Make greater in size, number, etc." , "get longer" , "Increase number of" , "Swell" .)