Complex sentence: The Muslims were able to hold them for some time, but the Persians pressed on, the vaguely retreating Muslim army was to hold the attack until further instructions from Khalid ibn Walid.
Split into simpler sentences: The Muslims were able to hold them for some time, but the Persians pressed on. The vaguely retreating Muslim army had to hold back the attack until further instructions from Khalid ibn Walid.