Spend regularly on uniform that's second-hand (4)
I believe the answer is:
'second-hand' is the definition.
(I've seen this before)
'spend regularly on uniform' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'on' is a charade indicator (letters next to each other).
'uniform' becomes 'u' (phonetic alphabet: alpha, bravo, charlie etc.).
The alternating letters of 'spend' are 'sed'.
'sed' after 'u' is 'USED'.
'that's' is the link.
(Other definitions for used that I've seen before include "with a previous owner" , "Took advantage of" , "Second-hand; exploited" , "Subject to exploitation" , "taken advantage of!" .)