As far as I know that there is no specific provision in the CLRA Act,1970 or the Rules made thereunder making it mandatory to commence the contract work only after issuance of the licence under the Act. If the work allotted on contract is not a prohibited activity u/s 10 of the Act, I am of the opinion that there can be simultaneous commencement of the work and submission of the licence application by the contractor.
You can start work with less than 20 workmen till obtaining the licence from the authority. In other case workmen can be engaged without licence after the deadline expired, if at all exist under the rule(deemed issue).
But engagement of labour 20 or more without licence is a violation.