The context is insufficient - I'm afraid that it is customary in Croatian to use "tajnik" (masculine) when implying a higher-ranking post (approx. eual to "commisioner" or "secretary" of the "general secretary type" ;)), whereas "tajnica" (feminine) usually stands for a PA.

I'd go for "pomoćnik tajnika" - vague enough, yet covering most ground and being a very close translation of the original text. Be careful though, this is masculine times 2 - female assistant is "pomoćnica", female secretary iz "tajnica" - thus it could also read "pomoćnica tajnice", "pomoćnik tajnice" or "pomoćnica tajnika".

(btw. ... authorized TO sign, I presume)