Guides in uniform and that lady upset me (10)
I believe the answer is:
usherettes
'guides' is the definition.
(I know this)
'uniform and that lady upset me' is the wordplay.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'and' means one lot of letters go next to another.
'that lady' becomes 'she' ('she' refers to a woman).
'upset' is a reversal indicator.
'me' becomes 'setter' (the person who set this crossword).
'setter' backwards is 'rettes'.
'u'+'she'+'rettes'='USHERETTES'
'in' acts as a link.
(Other definitions for usherettes that I've seen before include "Guides" , "female escorts" .)